1. What is a class?
Ans: The objects with the same data structure (attributes) and behavior (operations) are called class.
2. What is an object?
Ans: It is an entity which may correspond to real-world entities such as students, employees, bank
account. It may be concrete such as file system or conceptual such as scheduling policies in multiprocessor
operating system.
Every object will have data structures called attributes and behavior called operations.
3. What is the difference between an object and a class?
Ans: All objects possessing similar properties are grouped into class.
Example :–person is a class, ram, hari are objects of person class. All have similar attributes like name,
age, sex and similar operations like speak, walk.
Class person
{
private:
char name[20];
int age;
char sex;
public: speak();
walk();
};
4. What is the difference between class and structure?
Ans: In class the data members by default are private but in structure they are by default public
5. Define object based programming language?
Ans: Object based programming language support encapsulation and object identity without supporting
some important features of OOPs language.
Object based language=Encapsulation + object Identity
6. Define object oriented language?
Ans: Object-oriented language incorporates all the features of object based programming languages along
with inheritance and polymorphism.
Example: – c++, java.
7. Define OOPs?
Ans: OOP is a method of implementation in which programs are organized as co-operative collection of
objects, each of which represents an instance of some class and whose classes are all member of a
hierarchy of classes united through the property of inheritance.
8. What is public, protected, and private?
Ans: These are access specifier or a visibility lebels .The class member that has been declared as private
can be accessed only from within the class. Public members can be accessed from outside the class also.
Within the class or from the object of a class protected access limit is same as that of private but it plays a
prominent role in case of inheritance
9. What is a scope resolution operator?
Ans: The scope resolution operator permits a program to reference an identifier in the global scope that
has been hidden by another identifier with the same name in the local scope.
10. What do you mean by inheritance?
Ans: The mechanism of deriving a new class (derived) from an old class (base class) is called inheritance.
It allows the extension and reuse of existing code without having to rewrite the code from scratch.
11. What is abstraction?
Ans: The technique of creating user-defined data types, having the properties of built-in data types and a
set of permitted operators that are well suited to the application to be programmed is known as data
abstraction. Class is a construct for abstract data types (ADT).
12. What is encapsulation?
Ans: It is the mechanism that wraps the data and function it manipulates into single unit and keeps it safe
from external interference.
13. How variable declaration in c++ differs that in c?
Ans: C requires all the variables to be declared at the beginning of a scope but in c++ we can declare
variables anywhere in the scope. This makes the programmer easier to understand because the variables
are declared in the context of their use.
14. What are the c++ tokens?
Ans: c++ has the following tokens
I. keywords
II. Identifiers
III. Constants
IV. Strings
V. operators
15. What do you mean by reference variable in c++?
Ans: A reference variable provides an alias to a previously defined variable.
Data type & reference-name = variable name
16. What do you mean by implicit conversion?
Ans: Whenever data types are mixed in an expression then c++ performs the conversion automatically.
Here smaller type is converted to wider type.
Example- in case of integer and float integer is converted into float type.
17. What is the difference between method overloading and method overriding?
Ans: Overloading a method (or function) in C++ is the ability for functions of the same name to be defined
as long as these methods have different signatures (different set of parameters). Method overriding is the
ability of the inherited class rewriting the virtual method of the base class.
18. What are the defining traits of an object-oriented language?
The defining traits of an object-oriented language are:
encapsulation
inheritance
polymorphism
Ans:
Polymorphism: is a feature of OOPL that at run time depending upon the type of object the appropriate
method is called.
Inheritance: is a feature of OOPL that represents the “is a” relationship between different objects (classes).
Say in real life a manager is a employee. So in OOPL manger class is inherited from the employee class.
Encapsulation: is a feature of OOPL that is used to hide the information.
19. What is polymorphism?
Ans: Polymorphism is the idea that a base class can be inherited by several classes. A base class pointer
can point to its child class and a base class array can store different child class objects.
20. What do you mean by inline function?
Ans: An inline function is a function that is expanded inline when invoked.ie. the compiler replaces the
function call with the corresponding function code. An inline function is a function that is expanded in line
when it is invoked. That is the compiler replaces the function call with the corresponding function code
(similar to macro).
21 What is the difference between a NULL pointer and a void pointer?
Ans: A NULL pointer is a pointer of any type whose value is zero. A void pointer is a pointer to an object of
an unknown type, and is guaranteed to have enough bits to hold a pointer to any object. A void pointer is
not guaranteed to have enough bits to point to a function (though in general practice it does).
22. What is difference between C++ and Java?
Ans: C++ has pointers Java does not.
Java is platform independent C++ is not.
Java has garbage collection C++ does not.
23. What do you mean by multiple inheritance in C++ ?
Ans: Multiple inheritance is a feature in C++ by which one class can be of different types. Say class
teaching Assistant is inherited from two classes say teacher and Student.
24. What do you mean by virtual methods?
Ans: virtual methods are used to use the polymorphism feature in C++. Say class A is inherited from class
B. If we declare say function f() as virtual in class B and override the same function in class A then at
runtime appropriate method of the class will be called depending upon the type of the object.
25. What do you mean by static methods?
Ans: By using the static method there is no need creating an object of that class to use that method. We
can directly call that method on that class. For example, say class A has static function f(), then we can call
f() function as A.f(). There is no need of creating an object of class A.
26. How many ways are there to initialize an int with a constant?
Ans: Two.
There are two formats for initializers in C++ as shown in the example that follows. The first format uses the
traditional C notation. The second format uses constructor notation.
int foo = 123;
int bar (123);
27. What is a constructor?
Ans: Constructor is a special member function of a class, which is invoked automatically whenever an
instance of the class is created. It has the same name as its class.
28. What is destructor?
Ans: Destructor is a special member function of a class, which is invoked automatically whenever an object
goes out of the scope. It has the same name as its class with a tilde character prefixed.
29. What is an explicit constructor?
Ans: A conversion constructor declared with the explicit keyword. The compiler does not use an explicit
constructor to implement an implied conversion of types. It?s purpose is reserved explicitly for construction.
30 What is the Standard Template Library?
Ans: A library of container templates approved by the ANSI committee for inclusion in the standard C++
specification. A programmer who then launches into a discussion of the generic programming model,
iterators, allocators, algorithms, and such, has a higher than average understanding of the new technology
that STL brings to C++ programming.
31. What problem does the namespace feature solve?
Ans: Multiple providers of libraries might use common global identifiers causing a name collision when an
application tries to link with two or more such libraries. The namespace feature surrounds a library?s
external declarations with a unique namespace that eliminates the potential for those collisions. This
solution assumes that two library vendors don?t use the same namespace identifier, of course.
32. What is the use of ‘using’ declaration?
Ans: A using declaration makes it possible to use a name from a namespace
33. What is a template?
Ans: Templates allow us to create generic functions that admit any data type as parameters and return a
value without having to overload the function with all the possible data types. Until certain point they fulfill
the functionality of a macro. Its prototype is any of the two following ones:
template function_declaration;
template function_declaration;
34. Differentiate between a template class and class template?
Ans:
Template class:
A generic definition or a parameterized class not instantiated until the client provides the needed
information. It?s jargon for plain templates.
Class template:
A class template specifies how individual classes can be constructed much like the way a class specifies
how individual objects can be constructed. It?s jargon for plain classes.
35. What is the difference between a copy constructor and an overloaded assignment operator?
Ans: A copy constructor constructs a new object by using the content of the argument object. An
overloaded assignment operator assigns the contents of an existing object to another existing object of the
same class.
36. What is a virtual destructor?
Ans: The simple answer is that a virtual destructor is one that is declared with the virtual attribute.
37. What is an incomplete type?
Ans: Incomplete type refers to pointers in which there is non availability of the implementation of the
referenced location or it points to some location whose value is not available for modification.
Example:
int *i=0×400 // i points to address 400
*i=0; //set the value of memory location pointed by i.
Incomplete types are otherwise called uninitialized pointers.
38. What do you mean by Stack unwinding?
Ans: It is a process during exception handling when the destructor is called for all local objects between the
place where the exception was thrown and where it is caught.
39. What is a container class? What are the types of container classes?
Ans: A container class is a class that is used to hold objects in memory or external storage. A container
class acts as a generic holder. A container class has a predefined behavior and a well-known interface. A
container class is a supporting class whose purpose is to hide the topology used for maintaining the list of
objects in memory. When a container class contains a group of mixed objects, the container is called a
heterogeneous container; when the container is holding a group of objects that are all the same, the
container is called a homogeneous container
40. Name some pure object oriented languages?
Ans: Smalltalk, Java, Eiffel, Sather.
41. Name the operators that cannot be overloaded?
Ans: sizeof, ., .*, .->, ::, ?:
42. What is an adaptor class or Wrapper class?
Ans: A class that has no functionality of its own. Its member functions hide the use of a third party software
component or an object with the non-compatible interface or a non-object-oriented implementation.
43. What is a Null object?
Ans: It is an object of some class whose purpose is to indicate that a real object of that class does not
exist. One common use for a null object is a return value from a member function that is supposed to return
an object with some specified properties but cannot find such an object.
44. What is class invariant?
Ans: A class invariant is a condition that defines all valid states for an object. It is a logical condition to
ensure the correct working of a class. Class invariants must hold when an object is created, and they must
be preserved under all operations of the class. In particular all class invariants are both preconditions and
post-conditions for all operations or member functions of the class.
45. What is a dangling pointer?
Ans: A dangling pointer arises when you use the address of an object after its lifetime is over. This may
occur in situations like returning addresses of the automatic variables from a function or using the address
of the memory block after it is freed. Example: The following code snippet shows this:
class Sample
{
public:
int *ptr;
Sample(int i)
{
ptr = new int(i);
}
~Sample()
{
delete ptr;
}
void PrintVal()
{
cout
}
};
void SomeFunc(Sample x)
{
cout
}
int main()
{
Sample s1= 10;
SomeFunc(s1);
s1.PrintVal();
}
In the above example when PrintVal() function is called it is called by the pointer that has been freed by the
destructor in SomeFunc.
46. Differentiate between the message and method?
Ans:
Message:
Objects communicate by sending messages to each other.
A message is sent to invoke a method.
Method
Provides response to a message and it is an implementation of an operation
47. How can we access protected and private members of a class?
Ans: In the case of members protected and private, these could not be accessed from outside the same
class at which they are declared. This rule can be transgressed with the use of the friend keyword in a
class, so we can allow an external function to gain access to the protected and private members of a class.
48. Can you handle exception in C++?
Ans: Yes we can handle exception in C++ using keyword: try, catch and throw. Program statements that
we want to monitor for exceptions are contained in a try block. If an exception occurs within the try block, it
is thrown (using throw).The exception is caught, using catch, and processed.
49. What is virtual function?
Ans: A virtual function is a member function that is declared within a base class and
redefined by a derived class .To create a virtual function, the function declaration in the base class is
preceded by the keyword virtual.
50. What do you mean by early binding?
Ans:Early binding refers to the events that occur at compile time. Early binding occurs when
all information needed to call a function is known at compile time. Examples of early binding include normal
function calls, overloaded function calls, and overloaded operators. The advantage of early binding is
efficiency.
51. What do you mean by late binding?
Ans: Late binding refers to function calls that are not resolved until run time. Virtual functions are used to
achieve late binding. When access is via a base pointer or reference, the virtual function actually called is
determined by the type of object pointed to by the pointer.
other important question
Q: Is it possible to have Virtual Constructor? If yes, how? If not, Why not possible?
A: There is nothing like Virtual Constructor. The Constructor can?t be virtual as the constructor
is a code which is responsible for creating an instance of a class and it can?t be delegated to
any other object by virtual keyword means.
Q: What is constructor or ctor?
A: Constructor creates an object and initializes it. It also creates vtable for virtual functions. It is
different from other methods in a class.
Q: What about Virtual Destructor?
A: Yes there is a Virtual Destructor. A destructor can be virtual as it is possible as at runtime
depending on the type of object caller is calling to, proper destructor will be called.
Q: What is the difference between a copy constructor and an overloaded assignment operator?
A: A copy constructor constructs a new object by using the content of the argument object. An
overloaded assignment operator assigns the contents of an existing object to another existing
object of the same class.
Q: Can a constructor throws an exception? How to handle the error when the constructor fails?
A:The constructor never throws an error.
Q: What is default constructor?
A: Constructor with no arguments or all the arguments has default values.
Q: What is copy constructor?
A: Constructor which initializes the it's object member variables ( by
shallow copying) with another object of the same class. If you don't implement one in your class
then compiler implements one for you. for example:
(a) Boo Obj1(10); // calling Boo constructor
(b) Boo Obj2(Obj1); // calling boo copy constructor
(c) Boo Obj2 = Obj1;// calling boo copy constructor
Q: When are copy constructors called?
A: Copy constructors are called in following cases:
(a) when a function returns an object of that class by value
(b) when the object of that class is passed by value as an argument to a function
(c) when you construct an object based on another object of the same class
(d) When compiler generates a temporary object
Q: Can a copy constructor accept an object of the same class as parameter, instead of reference
of the object?
A: No. It is specified in the definition of the copy constructor itself. It should generate an error if
a programmer specifies a copy constructor with a first argument that is an object and not a
reference.
Q: What is conversion constructor?
A: constructor with a single argument makes that constructor as conversion ctor and it can be
used for type conversion.
for example:
class Boo
{
public:
Boo( int i );
};
Boo BooObject = 10 ; // assigning int 10 Boo object
Q:What is conversion operator??
A:class can have a public method for specific data type conversions.
for example:
class Boo
{
double value;
public:
Boo(int i )
operator double()
{
return value;
}
};
Boo BooObject;
double i = BooObject; // assigning object to variable i of type double.
now conversion operator gets called to assign the value.
Q: How can I handle a constructor that fails?
A: throw an exception. Constructors don't have a return type, so it's not possible to use return
codes. The best way to signal constructor failure is therefore to throw an exception.
Q: How can I handle a destructor that fails?
A: Write a message to a log-_le. But do not throw an exception. The C++ rule is that you must
never throw an exception from a destructor that is being called during the "stack unwinding"
process of another exception. For example, if someone says throw Foo(), the stack will be
unwound so all the stack frames between the throw Foo() and the } catch (Foo e) { will get
popped. This is called stack unwinding. During stack unwinding, all the local objects in all
those stack frames are destructed. If one of those destructors throws an exception (say it throws a
Bar object), the C++ runtime system is in a no-win situation: should it ignore the Bar and end up
in the } catch (Foo e) { where it was originally headed? Should it ignore the Foo and look for a }
catch (Bare) { handler? There is no good answer:either choice
loses information. So the C++ language guarantees that it will call terminate() at this point, and
terminate() kills the process. Bang you're dead.
Q: What is Virtual Destructor?
A: Using virtual destructors, you can destroy objects without knowing their type - the correct
destructor for the object is invoked using the virtual function mechanism. Note that destructors
can also be declared as pure virtual functions for abstract classes. if someone will derive from
your class, and if someone will say "new Derived", where "Derived" is derived from your class,
and if someone will say delete p, where the actual object's type is "Derived" but the pointer p's
type is your class.
Q: Can a copy constructor accept an object of the same class as parameter, instead of reference
of the object?
A: No. It is specified in the definition of the copy constructor itself. It should generate an error if
a programmer specifies a copy constructor with a first argument that is an object and not a
reference.
Q: What's the order that local objects are destructed?
A: In reverse order of construction: First constructed, last destructed.
In the following example, b's destructor will be executed first, then a's destructor:
void userCode()
{
Fred a;
Fred b;
...
}
Q: What's the order that objects in an array are destructed?
A: In reverse order of construction: First constructed, last destructed.
In the following example, the order for destructors will be a[9], a[8], ..., a[1], a[0]:
void userCode()
{
Fred a[10];
...
}
Q: Can I overload the destructor for my class?
A: No.
You can have only one destructor for a class Fred. It's always called Fred::~Fred(). It never takes
any parameters, and it never returns anything.
You can't pass parameters to the destructor anyway, since you never explicitly call a destructor
(well, almost never).
Q: Should I explicitly call a destructor on a local variable?
A: No!
The destructor will get called again at the close } of the block in which the local was created.
This is a guarantee of the language; it happens automagically; there's no way to stop it from
happening. But you can get really bad results from calling a destructor on the same object a
second time! Bang! You're dead!
Q: What if I want a local to "die" before the close } of the scope in which it was created? Can I
call a destructor on a local if I really want to?
A: No! [For context, please read the previous FAQ].
Suppose the (desirable) side effect of destructing a local File object is to close the File. Now
suppose you have an object f of a class File and you want File f to be closed before the end of the
scope (i.e., the }) of the scope of object f:
void someCode()
{
File f;
...insert code that should execute when f is still open...
We want the side-effect of f's destructor here!
...insert code that should execute after f is closed...
}
There is a simple solution to this problem. But in the mean time, remember: Do not explicitly
call the destructor!
Q: OK, OK already; I won't explicitly call the destructor of a local; but how do I handle the
above situation?
A: Simply wrap the extent of the lifetime of the local in an artificial block {...}:
void someCode()
{
{
File f;
...insert code that should execute when f is still open...
} f's destructor will automagically be called here!
...insert code here that should execute after f is closed...}
Q: What if I can't wrap the local in an artificial block?
A: Most of the time, you can limit the lifetime of a local by wrapping the local in an artificial
block ({...}). But if for some reason you can't do that, add a member function that has a similar
effect as the destructor. But do not call the destructor itself!
For example, in the case of class File, you might add a close() method. Typically the destructor
will simply call this close() method. Note that the close() method will need to mark the File
object so a subsequent call won't re-close an already-closed File. E.g., it might set the
fileHandle_ data member to some nonsensical value such as -1, and it might check at the
beginning to see if the fileHandle_ is already equal to -1:
class File {
public:
void close();
~File();
...
private:
int fileHandle_; // fileHandle_ >= 0 if/only-if it's open
};
File::~File()
{
close();
}
void File::close()
{
if (fileHandle_ >= 0) {
...insert code to call the OS to close the file...
fileHandle_ = -1;
}
}
Note that the other File methods may also need to check if the fileHandle_ is -1 (i.e., check if the
File is closed).
Note also that any constructors that don't actually open a file should set fileHandle_ to -1.
Q: But can I explicitly call a destructor if I've allocated my object with new?
A: Probably not.
Unless you used placement new, you should simply delete the object rather than explicitly
calling the destructor. For example, suppose you allocated the object via a typical new
expression:
Fred* p = new Fred();
Then the destructor Fred::~Fred() will automagically get called when you delete it via:
delete p; // Automagically calls p->~Fred()
You should not explicitly call the destructor, since doing so won't release the memory that was
allocated for the Fred object itself. Remember: delete p does two things: it calls the destructor
and it deallocates the memory.
Q: What is "placement new" and why would I use it?
A: There are many uses of placement new. The simplest use is to place an object at a particular
location in memory. This is done by supplying the place as a pointer parameter to the new part of
a new expression:
#include // Must #include this to use "placement new"
#include "Fred.h" // Declaration of class Fred
void someCode()
{
char memory[sizeof(Fred)]; // Line #1
void* place = memory; // Line #2
Fred* f = new(place) Fred(); // Line #3 (see "DANGER" below)
// The pointers f and place will be equal
...
}
Line #1 creates an array of sizeof(Fred) bytes of memory, which is big enough to hold a Fred
object. Line #2 creates a pointer place that points to the first byte of this memory (experienced C
programmers will note that this step was unnecessary; it's there only to make the code more
obvious). Line #3 essentially just calls the constructor Fred::Fred(). The this pointer in the Fred
constructor will be equal to place. The returned pointer f will therefore be equal to place.
ADVICE: Don't use this "placement new" syntax unless you have to. Use it only when you really
care that an object is placed at a particular location in memory. For example, when your
hardware has a memory-mapped I/O timer device, and you want to place a Clock object at that
memory location.
DANGER: You are taking sole responsibility that the pointer you pass to the "placement new"
operator points to a region of memory that is big enough and is properly aligned for the object
type that you're creating. Neither the compiler nor the run-time system make any attempt to
check whether you did this right. If your Fred class needs to be aligned on a 4 byte boundary but
you supplied a location that isn't properly aligned, you can have a serious disaster on your hands
(if you don't know what "alignment" means, please don't use the placement new syntax). You
have been warned.
You are also solely responsible for destructing the placed object. This is done by explicitly
calling the destructor:
void someCode()
{
char memory[sizeof(Fred)];
void* p = memory;
Fred* f = new(p) Fred();
...
f->~Fred(); // Explicitly call the destructor for the placed object
}
This is about the only time you ever explicitly call a destructor.
Note: there is a much cleaner but more sophisticated way of handling the destruction / deletion
situation.
Q: When I write a destructor, do I need to explicitly call the destructors for my member objects?
A: No. You never need to explicitly call a destructor (except with placement new).
A class's destructor (whether or not you explicitly define one) automagically invokes the
destructors for member objects. They are destroyed in the reverse order they appear within the
declaration for the class.
class Member {
public:
~Member();
...
};
class Fred {
public:
~Fred();
...
private:
Member x_;
Member y_;
Member z_;
};
Fred::~Fred()
{
// Compiler automagically calls z_.~Member()
// Compiler automagically calls y_.~Member()
// Compiler automagically calls x_.~Member()
}
Q: When I write a derived class's destructor, do I need to explicitly call the destructor for my
base class?
A: No. You never need to explicitly call a destructor (except with placement new).
A derived class's destructor (whether or not you explicitly define one) automagically invokes the
destructors for base class subobjects. Base classes are destructed after member objects. In the
event of multiple inheritance, direct base classes are destructed in the reverse order of their
appearance in the inheritance list.
class Member {
public:
~Member();
...
};
class Base {
public:
virtual ~Base(); // A virtual destructor
...
};
class Derived : public Base {
public:
~Derived();
...
private:
Member x_;
};
Derived::~Derived()
{
// Compiler automagically calls x_.~Member()
// Compiler automagically calls Base::~Base()
}
Note: Order dependencies with virtual inheritance are trickier. If you are relying on order
dependencies in a virtual inheritance hierarchy, you'll need a lot more information than is in this
FAQ.
Q: Is there any difference between List x; and List x();?
A: A big difference!
Suppose that List is the name of some class. Then function f() declares a local List object called
x:
void f()
{
List x; // Local object named x (of class List)
...
}
But function g() declares a function called x() that returns a List:
void g()
{
List x(); // Function named x (that returns a List)
...
}
Q: Can one constructor of a class call another constructor of the same class to initialize the this
object?
A: Nope.
Let's work an example. Suppose you want your constructor Foo::Foo(char) to call another
constructor of the same class, say Foo::Foo(char,int), in order that Foo::Foo(char,int) would help
initialize the this object. Unfortunately there's no way to do this in C++.
Some people do it anyway. Unfortunately it doesn't do what they want. For example, the line
Foo(x, 0); does not call Foo::Foo(char,int) on the this object. Instead it calls Foo::Foo(char,int) to
initialize a temporary, local object (not this), then it immediately destructs that temporary when
control flows over the ;.
class Foo {
public:
Foo(char x);
Foo(char x, int y);
...
};
Foo::Foo(char x)
{
...
Foo(x, 0); // this line does NOT help initialize the this object!!
...
}
You can sometimes combine two constructors via a default parameter:
class Foo {
public:
Foo(char x, int y=0); // this line combines the two constructors
...
};
If that doesn't work, e.g., if there isn't an appropriate default parameter that combines the two
constructors, sometimes you can share their common code in a private init() member function:
class Foo {
public:
Foo(char x);
Foo(char x, int y);
...
private:
void init(char x, int y);
};
Foo::Foo(char x)
{
init(x, int(x) + 7);
...
}
Foo::Foo(char x, int y)
{
init(x, y);
...
}
void Foo::init(char x, int y)
{
...
}
Q: Is the default constructor for Fred always Fred::Fred()?
A: No. A "default constructor" is a constructor that can be called with no arguments. One
example of this is a constructor that takes no parameters:
class Fred {
public:
Fred(); // Default constructor: can be called with no args
...
};
Another example of a "default constructor" is one that can take arguments, provided they are
given default values:
class Fred {
public:
Fred(int i=3, int j=5); // Default constructor: can be called with no args
...
};
Q: Should my constructors use "initialization lists" or "assignment"?
A: Initialization lists. In fact, constructors should initialize as a rule all member objects in the
initialization list. One exception is discussed further down.
Consider the following constructor that initializes member object x_ using an initialization list:
Fred::Fred() : x_(whatever) { }. The most common benefit of doing this is improved
performance. For example, if the expression whatever is the same type as member variable x_,
the result of the whatever expression is constructed directly inside x_ the compiler does not
make a separate copy of the object. Even if the types are not the same, the compiler is usually
able to do a better job with initialization lists than with assignments.
The other (inefficient) way to build constructors is via assignment, such as: Fred::Fred() { x_ =
whatever; }. In this case the expression whatever causes a separate, temporary object to be
created, and this temporary object is passed into the x_ object's assignment operator. Then that
temporary object is destructed at the ;. That's inefficient.
As if that wasn't bad enough, there's another source of inefficiency when using assignment in a
constructor: the member object will get fully constructed by its default constructor, and this
might, for example, allocate some default amount of memory or open some default file. All this
work could be for naught if the whatever expression and/or assignment operator causes the
object to close that file and/or release that memory (e.g., if the default constructor didn't allocate
a large enough pool of memory or if it opened the wrong file).
Conclusion: All other things being equal, your code will run faster if you use initialization lists
rather than assignment.
Note: There is no performance difference if the type of x_ is some built-in/intrinsic type, such as
int or char* or float. But even in these cases, my personal preference is to set those data members
in the initialization list rather than via assignment for consistency. Another symmetry argument
in favor of using initialization lists even for built-in/intrinsic types: non-static const and nonstatic
reference data members can't be assigned a value in the constructor, so for symmetry it
makes sense to initialize everything in the initialization list.
Now for the exceptions. Every rule has exceptions (hmmm; does "every rule has exceptions"
have exceptions? reminds me of Gdel's Incompleteness Theorems), and there are a couple of
exceptions to the "use initialization lists" rule. Bottom line is to use common sense: if it's
cheaper, better, faster, etc. to not use them, then by all means, don't use them. This might happen
when your class has two constructors that need to initialize the this object's data members in
different orders. Or it might happen when two data members are self-referential. Or when a datamember
needs a reference to the this object, and you want to avoid a compiler warning about
using the this keyword prior to the { that begins the constructor's body (when your particular
compiler happens to issue that particular warning). Or when you need to do an if/throw test on a
variable (parameter, global, etc.) prior to using that variable to initialize one of your this
members. This list is not exhaustive; please don't write me asking me to add another "Or
when...". The point is simply this: use common sense.
Q: Should you use the this pointer in the constructor?
A: Some people feel you should not use the this pointer in a constructor because the object is not
fully formed yet. However you can use this in the constructor (in the {body} and even in the
initialization list) if you are careful.
Here is something that always works: the {body} of a constructor (or a function called from the
constructor) can reliably access the data members declared in a base class and/or the data
members declared in the constructor's own class. This is because all those data members are
guaranteed to have been fully constructed by the time the constructor's {body} starts executing.
Here is something that never works: the {body} of a constructor (or a function called from the
constructor) cannot get down to a derived class by calling a virtual member function that is
overridden in the derived class. If your goal was to get to the overridden function in the derived
class, you won't get what you want. Note that you won't get to the override in the derived class
independent of how you call the virtual member function: explicitly using the this pointer (e.g.,
this->method()), implicitly using the this pointer (e.g., method()), or even calling some other
function that calls the virtual member function on your this object. The bottom line is this: even
if the caller is constructing an object of a derived class, during the constructor of the base class,
your object is not yet of that derived class. You have been warned.
Here is something that sometimes works: if you pass any of the data members in this object to
another data member's initializer, you must make sure that the other data member has already
been initialized. The good news is that you can determine whether the other data member has (or
has not) been initialized using some straightforward language rules that are independent of the
particular compiler you're using. The bad news it that you have to know those language rules
(e.g., base class sub-objects are initialized first (look up the order if you have multiple and/or
virtual inheritance!), then data members defined in the class are initialized in the order in which
they appear in the class declaration). If you don't know these rules, then don't pass any data
member from the this object (regardless of whether or not you explicitly use the this keyword) to
any other data member's initializer! And if you do know the rules, please be careful.
Q: What is the "Named Constructor Idiom"?
A: A technique that provides more intuitive and/or safer construction operations for users of your
class.
The problem is that constructors always have the same name as the class. Therefore the only way
to differentiate between the various constructors of a class is by the parameter list. But if there
are lots of constructors, the differences between them become somewhat subtle and error prone.
With the Named Constructor Idiom, you declare all the class's constructors in the private or
protected sections, and you provide public static methods that return an object. These static
methods are the so-called "Named Constructors." In general there is one such static method for
each different way to construct an object.
Q: What is virtual function?
A: When derived class overrides the base class method by redefining the same function, then if
client wants to access redefined the method from derived class through a pointer from base class
object, then you must define this function in base class as virtual function.
class parent
{
void Show()
{
cout << "i'm parent" << endl;
}
};
class child: public parent
{
void Show()
{
cout << "i'm child" << endl;
}
};
parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls parent->show()
now we goto virtual world...
class parent
{
virtual void Show()
{
cout << "i'm parent" << endl;
}
};
class child: public parent
{
void Show()
{
cout << "i'm child" << endl;
}
};
parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls child->show()
Q: What is a "pure virtual" member function?
A: The abstract class whose pure virtual method has to be implemented by all the classes which
derive on these. Otherwise it would result in a compilation error. This construct should be used
when one wants to ensure that all the derived classes implement the method defined as pure
virtual in base class.
Q: How virtual functions are implemented C++?
A: Virtual functions are implemented using a table of function pointers, called the vtable. There
is one entry in the table per virtual function in the class. This table is created by the constructor
of the class. When a derived class is constructed, its base class is constructed _rst which creates
the vtable. If the derived class overrides any of the base classes virtual functions, those entries in
the vtable are overwritten by the derived class constructor. This is why you should never call
virtual functions from a constructor: because the vtable entries for the object may not have
been set up by the derived class constructor yet, so you might end up calling base class
implementations of those virtual functions
Q: What is pure virtual function? or what is abstract class?
A: When you de_ne only function prototype in a base class without implementation and do the
complete implementation in derived class. This base class is called abstract class and client won't
able to instantiate an object using this base class. You can make a pure virtual function or
abstract class this way..
class Boo
{
void foo() = 0;
}
Boo MyBoo; // compilation error
Q: What is Pure Virtual Function? Why and when it is used?
A: The abstract class whose pure virtual method has to be implemented by all the classes which
derive on these. Otherwise it would result in a compilation error. This construct should be used
when one wants to ensure that all the derived classes implement the method defined as pure
virtual in base class.
Q: How Virtual functions call up is maintained?
A: Through Look up tables added by the compile to every class image. This also leads to
performance penalty.
Q: What is a virtual destructor?
A: The simple answer is that a virtual destructor is one that is declared with the virtual attribute.
The behavior of a virtual destructor is what is important. If you destroy an object through a caller
or reference to a base class, and the base-class destructor is not virtual, the derived-class
destructors are not executed, and the destruction might not be complete.
_ __ __ _ __ _ __ _
Q: What is inheritance?
A: Inheritance allows one class to reuse the state and behavior of another class. The derived class
inherits the properties and method implementations of the base class and extends it by overriding
methods and adding additional properties and methods.
Q: When should you use multiple inheritance?
A:There are three acceptable answers:- "Never,""Rarely," and "When the problem domain
cannot be accurately modeled any other way." Consider an Asset class, Building class, Vehicle
class, and CompanyCar class. All company cars are vehicles. Some company cars are assets
because the organizations own them. Others might be leased. Not all assets are vehicles. Money
accounts are assets. Real estate holdings are assets. Some real estate holdings are buildings. Not
all buildings are assets. Ad infinitum. When you diagram these relationships, it becomes
apparent that multiple inheritance is a likely and intuitive way to model this common problem
domain. The applicant should understand, however, that multiple inheritance, like a chainsaw, is
a useful tool that has its perils, needs respect, and is best avoided except when nothing else will
do.
Q: Explain the ISA and HASA class relationships. How would you implement each in a class
design?
A: A specialized class "is" a specialization of another class and, therefore, has the ISA
relationship with the other class. This relationship is best implemented by embedding an object
of the Salary class in the Employee class.
Q: When is a template a better solution than a base class?
A: When you are designing a generic class to contain or otherwise manage objects of other
types, when the format and behavior of those other types are unimportant to their containment or
management, and particularly when those other types are unknown (thus, the generality) to the
designer of the container or manager class.
Q: What is multiple inheritance(virtual inheritance)? What are its advantages and disadvantages?
A: Multiple Inheritance is the process whereby a child can be derived from more than one parent
class. The advantage of multiple inheritance is that it allows a class to inherit the functionality of
more than one base class thus allowing for modeling of complex relationships.
The disadvantage of multiple inheritance is that it can lead to a lot of confusion(ambiguity)
when two base classes implement a method with the same name.
Q: What a derived class inherits or doesn't inherit?
A: Inherits:
Every data member defined in the parent class (although such members may not always be
accessible in the derived class!)
Every ordinary member function of the parent class (although such members may not always be
accessible in the derived class!)
The same initial data layout as the base class.
Doesn't Inherit :
The base class's constructors and destructor.
The base class's assignment operator.
The base class's friends
_ __ _ _ __ _ _ _ __ _
Q: What is Polymorphism??
A: Polymorphism allows a client to treat di_erent objects in the same way even if they were
created from di_erent classes and exhibit di_erent behaviors. You can use implementation
inheritance to achieve polymorphism in languages such as C++ and Java. Base class object's
pointer can invoke methods in derived class objects. You can also achieve polymorphism in C++
by function overloading and operator overloading.
Q: What is problem with Runtime type identification?
A: The run time type identification comes at a cost of performance penalty. Compiler maintains
the class.
Q: What is Polymorphism?
A: Polymorphism allows a client to treat different objects in the same way even if they were
created from different classes and exhibit different behaviors. You can use implementation
inheritance to achieve polymorphism in languages such as C++ and Java. Base class object's
pointer can invoke methods in derived class objects. You can also achieve polymorphism in
C++ by function overloading and operator overloading.
__ _ _____
Q: What is a class?
A: A class is an expanded concept of a data structure: instead of holding only data, it can hold
both data and functions.
Q: What are the differences between a C++ struct and C++ class?
A: The default member and base class access specifies are different. This is one of the
commonly misunderstood aspects of C++. Believe it or not, many programmers think that a C++
struct is just like a C struct, while a C++ class has inheritance, access specifes, member
functions, overloaded operators, and so on. Actually, the C++ struct has all the features of the
class. The only differences are that a struct defaults to public member access and public base
class inheritance, and a class defaults to the private access specified and private base-class
inheritance.
Q: How do you know that your class needs a virtual destructor?
A: If your class has at least one virtual function, you should make a destructor for this class
virtual. This will allow you to delete a dynamic object through a caller to a base class object. If
the destructor is non-virtual, then wrong destructor will be invoked during deletion of the
dynamic object.
Q: What is encapsulation?
A: Containing and hiding Information about an object, such as internal data structures and code.
Encapsulation isolates the internal complexity of an object's operation from the rest of the
application. For example, a client component asking for net revenue from a business object need
not know the data's origin.
Q: What is "this" pointer?
A: The this pointer is a pointer accessible only within the member functions of a class, struct, or
union type. It points to the object for which the member function is called. Static member
functions do not have a this pointer. When a nonstatic member function is called for an
object, the address of the object is passed as a hidden argument to the function. For example, the
following function call
myDate.setMonth( 3 );
can be interpreted this way:
setMonth( &myDate, 3 );
The object's address is available from within the member function as the this pointer. It is legal,
though unnecessary, to use the this pointer when referring to members of the class.
Q: What happens when you make call "delete this;"?
A: The code has two built-in pitfalls. First, if it executes in a member function for an extern,
static, or automatic object, the program will probably crash as soon as the delete statement
executes. There is no portable way for an object to tell that it was instantiated on the heap, so the
class cannot assert that its object is properly instantiated. Second, when an object commits
suicide this way, the using program might not know about its demise. As far as the instantiating
program is concerned, the object remains in scope and continues to exist even though the object
did itself in. Subsequent dereferencing of the pointer can and usually does lead to disaster.
You should never do this. Since compiler does not know whether the object was allocated on the
stack or on the heap, "delete this" could cause a disaster.
Q: What is assignment operator?
A: Default assignment operator handles assigning one object to another of the same class.
Member to member copy (shallow copy)
Q: What are all the implicit member functions of the class? Or what are all the functions which
compiler implements for us if we don't define one?
A:
(a) default ctor
(b) copy ctor
(c) assignment operator
(d) default destructor
(e) address operator
Q: What is a container class? What are the types of container classes?
A: A container class is a class that is used to hold objects in memory or external storage. A
container class acts as a generic holder. A container class has a predefined behavior and a wellknown
interface. A container class is a supporting class whose purpose is to hide the topology
used for maintaining the list of objects in memory. When a container class contains a group of
mixed objects, the container is called a heterogeneous container; when the container is holding a
group of objects that are all the same, the container is called a homogeneous container.
Q: What is Overriding?
A: To override a method, a subclass of the class that originally declared the method must declare
a method with the same name, return type (or a subclass of that return type), and same parameter
list.
The definition of the method overriding is:
· Must have same method name.
· Must have same data type.
· Must have same argument list.
Overriding a method means that replacing a method functionality in child class. To imply
overriding functionality we need parent and child classes. In the child class you define the same
method signature as one defined in the parent class.
Q: What is a nested class? Why can it be useful?
A: A nested class is a class enclosed within the scope of another class. For example:
// Example 1: Nested class
//
class OuterClass
{
class NestedClass
{
// ...
};
// ...
};
Nested classes are useful for organizing code and controlling access and dependencies. Nested
classes obey access rules just like other parts of a class do; so, in Example 1, if NestedClass is
public then any code can name it as OuterClass::NestedClass. Often nested classes contain
private implementation details, and are therefore made private; in Example 1, if NestedClass
is private, then only OuterClass's members and friends can use NestedClass. When you
instantiate as outer class, it won't instantiate inside class.
Q: What is a local class? Why can it be useful?
A: Local class is a class defined within the scope of a function _ any function, whether a
member function or a free function. For example:
// Example 2: Local class
//
int f()
{
class LocalClass
{
// ...
};
// ...
};
Like nested classes, local classes can be a useful tool for managing code dependencies.
Q: What a derived class can add?
A: New data members
New member functions
New constructors and destructor
New friends
Q: What happens when a derived-class object is created and destroyed?
A: Space is allocated (on the stack or the heap) for the full object (that is, enough space to store
the data members inherited from the base class plus the data members defined in the derived
class itself)
The base class's constructor is called to initialize the data members inherited from the base class
The derived class's constructor is then called to initialize the data members added in the derived
class
The derived-class object is then usable
When the object is destroyed (goes out of scope or is deleted) the derived class's destructor is
called on the object first
Then the base class's destructor is called on the object
Finally the allocated space for the full object is reclaimed
Q: What is the difference between new/delete and malloc/free?
A: Malloc/free do not know about constructors and destructors. New and delete create and
destroy objects, while malloc and free allocate and deallocate memory.
Q:What is difference between new and malloc?
A: Both malloc and new functions are used for dynamic memory allocations and the basic
difference is: malloc requires a special "typecasting" when it allocates memory for eg. if the
pointer used is the char pointer then after the processor allocates memory then this allocated
memory needs to be typecasted to char pointer i.e (char*).but new does not requires any
typecasting. Also, free is the keyword used to free the memory while using malloc and delete the
keyword to free memory while using new, otherwise this will lead the memory leak.
Q: What is the difference between delete and delete[]?
A: Whenever you allocate memory with new[], you have to free the memory using delete[].
When you allocate memory with 'new', then use 'delete' without the brackets. You use new[] to
allocate an array of values (always starting at the index 0).
Q: What is difference between malloc()/free() and new/delete?
A: malloc allocates memory for object in heap but doesn't invoke object's constructor to initialize
the object. new allocates memory and also invokes constructor to initialize the object. malloc()
and free() do not support object semantics, does not construct and destruct objects
Q: What is the difference between "new" and "operator new" ?
A:"operator new" works like malloc.
Q: What is Memory alignment??
A: The term alignment primarily means the tendency of an address pointer value to be a multiple
of some power of two. So a pointer with two byte alignment has a zero in the least signi_cant bit.
And a pointer with four byte alignment has a zero in both the two least signi_cant bits. And so
on. More alignment means a longer sequence of zero bits in the lowest bits of a pointer.
Q: How does free know the size of memory to be deleted.?
int *i = (int *)malloc(12); followed by free(i); how did free function call know how much of
memory to delete?
A: It depends on the implementation, but there is usually a malloc header added to all the
memory allocated through malloc. on Linux its 4 bytes of memory preceding the memory
returned to you, whihc contains the number of bytes allocated + 4(itself).
so when you say,
int *i = (int *)malloc(12);
it allocates 16 bytes.
As you can see above total of 16 bytes are allocated, first 4 bytes stores the number of bytes
allocated(offset to the next memory from the start of this memory).
address of the 5th byte is returned to i. now i can access 12 bytes from this byte.
Q: Can I free() pointers allocated with new? Can I delete pointers allocated with malloc()?
A: No!
It is perfectly legal, moral, and wholesome to use malloc() and delete in the same program, or to
use new and free() in the same program. But it is illegal, immoral, and despicable to call free()
with a pointer allocated via new, or to call delete on a pointer allocated via malloc().
Beware! I occasionally get e-mail from people telling me that it works OK for them on machine
X and compiler Y. Just because they don't see bad symptoms in a simple test case doesn't mean it
won't crash in the field. Even if they know it won't crash on their particular compiler doesn't
mean it will work safely on another compiler, another platform, or even another version of the
same compiler.
Q: Why should I use new instead of trustworthy old malloc()?
A: Constructors/destructors, type safety, overridability.
Constructors/destructors: unlike malloc(sizeof(Fred)), new Fred() calls Fred's constructor.
Similarly, delete p calls *p's destructor.
Type safety: malloc() returns a void* which isn't type safe. new Fred() returns a pointer of the
right type (a Fred*).
Overridability: new is an operator that can be overridden by a class, while malloc() is not
overridable on a per-class basis.
Q: Can I use realloc() on pointers allocated via new?
A: No!
When realloc() has to copy the allocation, it uses a bitwise copy operation, which will tear many
C++ objects to shreds. C++ objects should be allowed to copy themselves. They use their own
copy constructor or assignment operator.
Besides all that, the heap that new uses may not be the same as the heap that malloc() and
realloc() use!
Q: What are C++ storage classes?
A:
auto: the default. Variables are automatically created and initialized when they are defined and
are destroyed at the end of the block containing their definition. They are not visible
outside that block.
register: a type of auto variable. a suggestion to the compiler to use a CPU register for
performance.
static: a variable that is known only in the function that contains its definition but is never
destroyed and retains its value between calls to that function. It exists from the time the
program begins execution.
extern: a static variable whose definition and placement is determined when all object and library
modules are combined (linked) to form the executable code file. It can be visible outside
the file where it is defined.
Q: What are storage qualifiers in C++ ?
A: They are:
Const: Indicates that memory once initialized, should not be altered by a program.
Volatile: Indicates that the value in the memory location can be altered even though nothing
in the program code modifies the contents. for example if you have a pointer to hardware
location that contains the time, where hardware changes the value of this pointer variable
and not the program. The intent of this keyword to improve the optimization ability of the
compiler.
Mutable: Iindicates that particular member of a structure or class can be altered even if a
particular structure variable, class, or class member function is constant.
Q: What is reference ??
A: reference is a name that acts as an alias, or alternative name, for a previously defined variable
or an object. prepending variable with "&" symbol makes it as reference.
for example:
int a;
int &b = a;
Q: What is passing by reference?
A: Method of passing arguments to a function which takes parameter of type reference.
for example:
void swap( int & x, int &amp;amp;amp; y )
{
int temp = x;
x = y;
y = temp;
}
int a=2, b=3;
swap( a, b );
Basically, inside the function there won't be any copy of the arguments "x" and "y" instead they
refer to original variables a and b. so no extra memory needed to pass arguments and it is more
efficient.
Q: When do use "const" reference arguments in function?
A:
a) Using const protects you against programming errors that inadvertently alter data.
b) Using const allows function to process both const and non-const actual arguments, while a
function without const in the prototype can only accept non constant arguments.
c) Using a const reference allows the function to generate and use a temporary variable
appropriately.
Q: When are temporary variables created by C++ compiler?
A: Provided that function parameter is a "const reference", compiler generates temporary
variable
in following2 ways.
a) The actual argument is the correct type, but it
isn't Lvalue
double Cube(const double & num)
{
num = num * num * num;
return num;
}
double temp = 2.0;
double value = cube(3.0 + temp); // argument is a expression and not a Lvalue;
b) The actual argument is of the wrong type, but of a type that can be converted to the correct
type
long temp = 3L;
double value = cuberoot ( temp); // long to double conversion
Q: What problem does the namespace feature solve?
A: Multiple providers of libraries might use common global identifiers causing a name collision
when an application tries to link with two or more such libraries. The namespace feature
surrounds a library's external declarations with a unique namespace that eliminates the potential
for those collisions.
namespace [identifier] { namespace-body }
A namespace declaration identifies and assigns a name to a declarative region.
The identifier in a namespace declaration must be unique in the declarative region in which it is
used. The identifier is the name of the namespace and is used to reference its members.
Q: What is the use of 'using' declaration?
A: A using declaration makes it possible to use a name from a namespace without the scope
operator.
Q: What is an Iterator class?
A: A class that is used to traverse through the objects maintained by a container class. There are
_ve categories of iterators: input iterators, output iterators, forward iterators, bidirectional
iterators, random access. An iterator is an entity that gives access to the contents of a container
object without violating encapsulation constraints. Access to the contents is granted on a one-atatime
basis in order. The order can be storage order (as in lists and queues) or some arbitrary
order (as in array indices) or according to some ordering relation (as in an ordered binary tree).
The iterator is a construct, which provides an interface that, when called, yields either the next
element in the container, or some value denoting the fact that there are no more elements to
examine. Iterators hide the details of access to and update of the elements of a container class.
Something like a pointer.
Q: What do you mean by Stack unwinding?
A: It is a process during exception handling when the destructor is called for all local objects in
the stack between the place where the exception was thrown and where it is caught.
Q: Implementation of string length using pointer hopping
A: #include
// Test to see if pointer hopping is worthwhile.
Q: What is inline function?
A: The inline keyword tells the compiler to substitute the code within the function de_nition for
every instance of a function call. However, substitution occurs only at the compiler's discretion.
For example, the compiler does not inline a function if its address is taken or if it is too large to
inline.
Q: What is name mangling in C++??
A: The process of encoding the parameter types with the function/method name into a unique
name is called name mangling. The inverse process is called demangling.
For example Foo::bar(int, long) const is mangled as 'bar__C3Fooil'.
For a constructor, the method name is left out.
That is Foo::Foo(int, long) const is mangled as „C3Fooil?
Q: Can you think of a situation where your program would crash without reaching the
breakpoint which you set at the beginning of main()?
A: C++ allows for dynamic initialization of global variables before main() is invoked. It is possible that initialization of global will invoke some function. If this function crashes the crash
will occur before main() is entered. Name two cases where you MUST use initialization
list as opposed to assignment in constructors. Both non-static const data members and reference
data members cannot be assigned values; instead, you should use initialization list to initialize
them.
Q: What is faster ++i or i++, where i is an interger variable?
A: The answer to this lies in the fact, how they used. With ++i(PreIncrement) the variable is
incremented and new value is returned. So it requires one instruction to increment the variable.
In case of i++(post Increment), the old value has to be returned or used in the expression and
then the variable is incrememented after the expression is evaluated. Since you need one
instruction to save the old value to be used in the expression and other instruction to increment
the variable, its comparatively slower.
Ans: The objects with the same data structure (attributes) and behavior (operations) are called class.
2. What is an object?
Ans: It is an entity which may correspond to real-world entities such as students, employees, bank
account. It may be concrete such as file system or conceptual such as scheduling policies in multiprocessor
operating system.
Every object will have data structures called attributes and behavior called operations.
3. What is the difference between an object and a class?
Ans: All objects possessing similar properties are grouped into class.
Example :–person is a class, ram, hari are objects of person class. All have similar attributes like name,
age, sex and similar operations like speak, walk.
Class person
{
private:
char name[20];
int age;
char sex;
public: speak();
walk();
};
4. What is the difference between class and structure?
Ans: In class the data members by default are private but in structure they are by default public
5. Define object based programming language?
Ans: Object based programming language support encapsulation and object identity without supporting
some important features of OOPs language.
Object based language=Encapsulation + object Identity
6. Define object oriented language?
Ans: Object-oriented language incorporates all the features of object based programming languages along
with inheritance and polymorphism.
Example: – c++, java.
7. Define OOPs?
Ans: OOP is a method of implementation in which programs are organized as co-operative collection of
objects, each of which represents an instance of some class and whose classes are all member of a
hierarchy of classes united through the property of inheritance.
8. What is public, protected, and private?
Ans: These are access specifier or a visibility lebels .The class member that has been declared as private
can be accessed only from within the class. Public members can be accessed from outside the class also.
Within the class or from the object of a class protected access limit is same as that of private but it plays a
prominent role in case of inheritance
9. What is a scope resolution operator?
Ans: The scope resolution operator permits a program to reference an identifier in the global scope that
has been hidden by another identifier with the same name in the local scope.
10. What do you mean by inheritance?
Ans: The mechanism of deriving a new class (derived) from an old class (base class) is called inheritance.
It allows the extension and reuse of existing code without having to rewrite the code from scratch.
11. What is abstraction?
Ans: The technique of creating user-defined data types, having the properties of built-in data types and a
set of permitted operators that are well suited to the application to be programmed is known as data
abstraction. Class is a construct for abstract data types (ADT).
12. What is encapsulation?
Ans: It is the mechanism that wraps the data and function it manipulates into single unit and keeps it safe
from external interference.
13. How variable declaration in c++ differs that in c?
Ans: C requires all the variables to be declared at the beginning of a scope but in c++ we can declare
variables anywhere in the scope. This makes the programmer easier to understand because the variables
are declared in the context of their use.
14. What are the c++ tokens?
Ans: c++ has the following tokens
I. keywords
II. Identifiers
III. Constants
IV. Strings
V. operators
15. What do you mean by reference variable in c++?
Ans: A reference variable provides an alias to a previously defined variable.
Data type & reference-name = variable name
16. What do you mean by implicit conversion?
Ans: Whenever data types are mixed in an expression then c++ performs the conversion automatically.
Here smaller type is converted to wider type.
Example- in case of integer and float integer is converted into float type.
17. What is the difference between method overloading and method overriding?
Ans: Overloading a method (or function) in C++ is the ability for functions of the same name to be defined
as long as these methods have different signatures (different set of parameters). Method overriding is the
ability of the inherited class rewriting the virtual method of the base class.
18. What are the defining traits of an object-oriented language?
The defining traits of an object-oriented language are:
encapsulation
inheritance
polymorphism
Ans:
Polymorphism: is a feature of OOPL that at run time depending upon the type of object the appropriate
method is called.
Inheritance: is a feature of OOPL that represents the “is a” relationship between different objects (classes).
Say in real life a manager is a employee. So in OOPL manger class is inherited from the employee class.
Encapsulation: is a feature of OOPL that is used to hide the information.
19. What is polymorphism?
Ans: Polymorphism is the idea that a base class can be inherited by several classes. A base class pointer
can point to its child class and a base class array can store different child class objects.
20. What do you mean by inline function?
Ans: An inline function is a function that is expanded inline when invoked.ie. the compiler replaces the
function call with the corresponding function code. An inline function is a function that is expanded in line
when it is invoked. That is the compiler replaces the function call with the corresponding function code
(similar to macro).
21 What is the difference between a NULL pointer and a void pointer?
Ans: A NULL pointer is a pointer of any type whose value is zero. A void pointer is a pointer to an object of
an unknown type, and is guaranteed to have enough bits to hold a pointer to any object. A void pointer is
not guaranteed to have enough bits to point to a function (though in general practice it does).
22. What is difference between C++ and Java?
Ans: C++ has pointers Java does not.
Java is platform independent C++ is not.
Java has garbage collection C++ does not.
23. What do you mean by multiple inheritance in C++ ?
Ans: Multiple inheritance is a feature in C++ by which one class can be of different types. Say class
teaching Assistant is inherited from two classes say teacher and Student.
24. What do you mean by virtual methods?
Ans: virtual methods are used to use the polymorphism feature in C++. Say class A is inherited from class
B. If we declare say function f() as virtual in class B and override the same function in class A then at
runtime appropriate method of the class will be called depending upon the type of the object.
25. What do you mean by static methods?
Ans: By using the static method there is no need creating an object of that class to use that method. We
can directly call that method on that class. For example, say class A has static function f(), then we can call
f() function as A.f(). There is no need of creating an object of class A.
26. How many ways are there to initialize an int with a constant?
Ans: Two.
There are two formats for initializers in C++ as shown in the example that follows. The first format uses the
traditional C notation. The second format uses constructor notation.
int foo = 123;
int bar (123);
27. What is a constructor?
Ans: Constructor is a special member function of a class, which is invoked automatically whenever an
instance of the class is created. It has the same name as its class.
28. What is destructor?
Ans: Destructor is a special member function of a class, which is invoked automatically whenever an object
goes out of the scope. It has the same name as its class with a tilde character prefixed.
29. What is an explicit constructor?
Ans: A conversion constructor declared with the explicit keyword. The compiler does not use an explicit
constructor to implement an implied conversion of types. It?s purpose is reserved explicitly for construction.
30 What is the Standard Template Library?
Ans: A library of container templates approved by the ANSI committee for inclusion in the standard C++
specification. A programmer who then launches into a discussion of the generic programming model,
iterators, allocators, algorithms, and such, has a higher than average understanding of the new technology
that STL brings to C++ programming.
31. What problem does the namespace feature solve?
Ans: Multiple providers of libraries might use common global identifiers causing a name collision when an
application tries to link with two or more such libraries. The namespace feature surrounds a library?s
external declarations with a unique namespace that eliminates the potential for those collisions. This
solution assumes that two library vendors don?t use the same namespace identifier, of course.
32. What is the use of ‘using’ declaration?
Ans: A using declaration makes it possible to use a name from a namespace
33. What is a template?
Ans: Templates allow us to create generic functions that admit any data type as parameters and return a
value without having to overload the function with all the possible data types. Until certain point they fulfill
the functionality of a macro. Its prototype is any of the two following ones:
template function_declaration;
template function_declaration;
34. Differentiate between a template class and class template?
Ans:
Template class:
A generic definition or a parameterized class not instantiated until the client provides the needed
information. It?s jargon for plain templates.
Class template:
A class template specifies how individual classes can be constructed much like the way a class specifies
how individual objects can be constructed. It?s jargon for plain classes.
35. What is the difference between a copy constructor and an overloaded assignment operator?
Ans: A copy constructor constructs a new object by using the content of the argument object. An
overloaded assignment operator assigns the contents of an existing object to another existing object of the
same class.
36. What is a virtual destructor?
Ans: The simple answer is that a virtual destructor is one that is declared with the virtual attribute.
37. What is an incomplete type?
Ans: Incomplete type refers to pointers in which there is non availability of the implementation of the
referenced location or it points to some location whose value is not available for modification.
Example:
int *i=0×400 // i points to address 400
*i=0; //set the value of memory location pointed by i.
Incomplete types are otherwise called uninitialized pointers.
38. What do you mean by Stack unwinding?
Ans: It is a process during exception handling when the destructor is called for all local objects between the
place where the exception was thrown and where it is caught.
39. What is a container class? What are the types of container classes?
Ans: A container class is a class that is used to hold objects in memory or external storage. A container
class acts as a generic holder. A container class has a predefined behavior and a well-known interface. A
container class is a supporting class whose purpose is to hide the topology used for maintaining the list of
objects in memory. When a container class contains a group of mixed objects, the container is called a
heterogeneous container; when the container is holding a group of objects that are all the same, the
container is called a homogeneous container
40. Name some pure object oriented languages?
Ans: Smalltalk, Java, Eiffel, Sather.
41. Name the operators that cannot be overloaded?
Ans: sizeof, ., .*, .->, ::, ?:
42. What is an adaptor class or Wrapper class?
Ans: A class that has no functionality of its own. Its member functions hide the use of a third party software
component or an object with the non-compatible interface or a non-object-oriented implementation.
43. What is a Null object?
Ans: It is an object of some class whose purpose is to indicate that a real object of that class does not
exist. One common use for a null object is a return value from a member function that is supposed to return
an object with some specified properties but cannot find such an object.
44. What is class invariant?
Ans: A class invariant is a condition that defines all valid states for an object. It is a logical condition to
ensure the correct working of a class. Class invariants must hold when an object is created, and they must
be preserved under all operations of the class. In particular all class invariants are both preconditions and
post-conditions for all operations or member functions of the class.
45. What is a dangling pointer?
Ans: A dangling pointer arises when you use the address of an object after its lifetime is over. This may
occur in situations like returning addresses of the automatic variables from a function or using the address
of the memory block after it is freed. Example: The following code snippet shows this:
class Sample
{
public:
int *ptr;
Sample(int i)
{
ptr = new int(i);
}
~Sample()
{
delete ptr;
}
void PrintVal()
{
cout
}
};
void SomeFunc(Sample x)
{
cout
}
int main()
{
Sample s1= 10;
SomeFunc(s1);
s1.PrintVal();
}
In the above example when PrintVal() function is called it is called by the pointer that has been freed by the
destructor in SomeFunc.
46. Differentiate between the message and method?
Ans:
Message:
Objects communicate by sending messages to each other.
A message is sent to invoke a method.
Method
Provides response to a message and it is an implementation of an operation
47. How can we access protected and private members of a class?
Ans: In the case of members protected and private, these could not be accessed from outside the same
class at which they are declared. This rule can be transgressed with the use of the friend keyword in a
class, so we can allow an external function to gain access to the protected and private members of a class.
48. Can you handle exception in C++?
Ans: Yes we can handle exception in C++ using keyword: try, catch and throw. Program statements that
we want to monitor for exceptions are contained in a try block. If an exception occurs within the try block, it
is thrown (using throw).The exception is caught, using catch, and processed.
49. What is virtual function?
Ans: A virtual function is a member function that is declared within a base class and
redefined by a derived class .To create a virtual function, the function declaration in the base class is
preceded by the keyword virtual.
50. What do you mean by early binding?
Ans:Early binding refers to the events that occur at compile time. Early binding occurs when
all information needed to call a function is known at compile time. Examples of early binding include normal
function calls, overloaded function calls, and overloaded operators. The advantage of early binding is
efficiency.
51. What do you mean by late binding?
Ans: Late binding refers to function calls that are not resolved until run time. Virtual functions are used to
achieve late binding. When access is via a base pointer or reference, the virtual function actually called is
determined by the type of object pointed to by the pointer.
other important question
Q: Is it possible to have Virtual Constructor? If yes, how? If not, Why not possible?
A: There is nothing like Virtual Constructor. The Constructor can?t be virtual as the constructor
is a code which is responsible for creating an instance of a class and it can?t be delegated to
any other object by virtual keyword means.
Q: What is constructor or ctor?
A: Constructor creates an object and initializes it. It also creates vtable for virtual functions. It is
different from other methods in a class.
Q: What about Virtual Destructor?
A: Yes there is a Virtual Destructor. A destructor can be virtual as it is possible as at runtime
depending on the type of object caller is calling to, proper destructor will be called.
Q: What is the difference between a copy constructor and an overloaded assignment operator?
A: A copy constructor constructs a new object by using the content of the argument object. An
overloaded assignment operator assigns the contents of an existing object to another existing
object of the same class.
Q: Can a constructor throws an exception? How to handle the error when the constructor fails?
A:The constructor never throws an error.
Q: What is default constructor?
A: Constructor with no arguments or all the arguments has default values.
Q: What is copy constructor?
A: Constructor which initializes the it's object member variables ( by
shallow copying) with another object of the same class. If you don't implement one in your class
then compiler implements one for you. for example:
(a) Boo Obj1(10); // calling Boo constructor
(b) Boo Obj2(Obj1); // calling boo copy constructor
(c) Boo Obj2 = Obj1;// calling boo copy constructor
Q: When are copy constructors called?
A: Copy constructors are called in following cases:
(a) when a function returns an object of that class by value
(b) when the object of that class is passed by value as an argument to a function
(c) when you construct an object based on another object of the same class
(d) When compiler generates a temporary object
Q: Can a copy constructor accept an object of the same class as parameter, instead of reference
of the object?
A: No. It is specified in the definition of the copy constructor itself. It should generate an error if
a programmer specifies a copy constructor with a first argument that is an object and not a
reference.
Q: What is conversion constructor?
A: constructor with a single argument makes that constructor as conversion ctor and it can be
used for type conversion.
for example:
class Boo
{
public:
Boo( int i );
};
Boo BooObject = 10 ; // assigning int 10 Boo object
Q:What is conversion operator??
A:class can have a public method for specific data type conversions.
for example:
class Boo
{
double value;
public:
Boo(int i )
operator double()
{
return value;
}
};
Boo BooObject;
double i = BooObject; // assigning object to variable i of type double.
now conversion operator gets called to assign the value.
Q: How can I handle a constructor that fails?
A: throw an exception. Constructors don't have a return type, so it's not possible to use return
codes. The best way to signal constructor failure is therefore to throw an exception.
Q: How can I handle a destructor that fails?
A: Write a message to a log-_le. But do not throw an exception. The C++ rule is that you must
never throw an exception from a destructor that is being called during the "stack unwinding"
process of another exception. For example, if someone says throw Foo(), the stack will be
unwound so all the stack frames between the throw Foo() and the } catch (Foo e) { will get
popped. This is called stack unwinding. During stack unwinding, all the local objects in all
those stack frames are destructed. If one of those destructors throws an exception (say it throws a
Bar object), the C++ runtime system is in a no-win situation: should it ignore the Bar and end up
in the } catch (Foo e) { where it was originally headed? Should it ignore the Foo and look for a }
catch (Bare) { handler? There is no good answer:either choice
loses information. So the C++ language guarantees that it will call terminate() at this point, and
terminate() kills the process. Bang you're dead.
Q: What is Virtual Destructor?
A: Using virtual destructors, you can destroy objects without knowing their type - the correct
destructor for the object is invoked using the virtual function mechanism. Note that destructors
can also be declared as pure virtual functions for abstract classes. if someone will derive from
your class, and if someone will say "new Derived", where "Derived" is derived from your class,
and if someone will say delete p, where the actual object's type is "Derived" but the pointer p's
type is your class.
Q: Can a copy constructor accept an object of the same class as parameter, instead of reference
of the object?
A: No. It is specified in the definition of the copy constructor itself. It should generate an error if
a programmer specifies a copy constructor with a first argument that is an object and not a
reference.
Q: What's the order that local objects are destructed?
A: In reverse order of construction: First constructed, last destructed.
In the following example, b's destructor will be executed first, then a's destructor:
void userCode()
{
Fred a;
Fred b;
...
}
Q: What's the order that objects in an array are destructed?
A: In reverse order of construction: First constructed, last destructed.
In the following example, the order for destructors will be a[9], a[8], ..., a[1], a[0]:
void userCode()
{
Fred a[10];
...
}
Q: Can I overload the destructor for my class?
A: No.
You can have only one destructor for a class Fred. It's always called Fred::~Fred(). It never takes
any parameters, and it never returns anything.
You can't pass parameters to the destructor anyway, since you never explicitly call a destructor
(well, almost never).
Q: Should I explicitly call a destructor on a local variable?
A: No!
The destructor will get called again at the close } of the block in which the local was created.
This is a guarantee of the language; it happens automagically; there's no way to stop it from
happening. But you can get really bad results from calling a destructor on the same object a
second time! Bang! You're dead!
Q: What if I want a local to "die" before the close } of the scope in which it was created? Can I
call a destructor on a local if I really want to?
A: No! [For context, please read the previous FAQ].
Suppose the (desirable) side effect of destructing a local File object is to close the File. Now
suppose you have an object f of a class File and you want File f to be closed before the end of the
scope (i.e., the }) of the scope of object f:
void someCode()
{
File f;
...insert code that should execute when f is still open...
We want the side-effect of f's destructor here!
...insert code that should execute after f is closed...
}
There is a simple solution to this problem. But in the mean time, remember: Do not explicitly
call the destructor!
Q: OK, OK already; I won't explicitly call the destructor of a local; but how do I handle the
above situation?
A: Simply wrap the extent of the lifetime of the local in an artificial block {...}:
void someCode()
{
{
File f;
...insert code that should execute when f is still open...
} f's destructor will automagically be called here!
...insert code here that should execute after f is closed...}
Q: What if I can't wrap the local in an artificial block?
A: Most of the time, you can limit the lifetime of a local by wrapping the local in an artificial
block ({...}). But if for some reason you can't do that, add a member function that has a similar
effect as the destructor. But do not call the destructor itself!
For example, in the case of class File, you might add a close() method. Typically the destructor
will simply call this close() method. Note that the close() method will need to mark the File
object so a subsequent call won't re-close an already-closed File. E.g., it might set the
fileHandle_ data member to some nonsensical value such as -1, and it might check at the
beginning to see if the fileHandle_ is already equal to -1:
class File {
public:
void close();
~File();
...
private:
int fileHandle_; // fileHandle_ >= 0 if/only-if it's open
};
File::~File()
{
close();
}
void File::close()
{
if (fileHandle_ >= 0) {
...insert code to call the OS to close the file...
fileHandle_ = -1;
}
}
Note that the other File methods may also need to check if the fileHandle_ is -1 (i.e., check if the
File is closed).
Note also that any constructors that don't actually open a file should set fileHandle_ to -1.
Q: But can I explicitly call a destructor if I've allocated my object with new?
A: Probably not.
Unless you used placement new, you should simply delete the object rather than explicitly
calling the destructor. For example, suppose you allocated the object via a typical new
expression:
Fred* p = new Fred();
Then the destructor Fred::~Fred() will automagically get called when you delete it via:
delete p; // Automagically calls p->~Fred()
You should not explicitly call the destructor, since doing so won't release the memory that was
allocated for the Fred object itself. Remember: delete p does two things: it calls the destructor
and it deallocates the memory.
Q: What is "placement new" and why would I use it?
A: There are many uses of placement new. The simplest use is to place an object at a particular
location in memory. This is done by supplying the place as a pointer parameter to the new part of
a new expression:
#include // Must #include this to use "placement new"
#include "Fred.h" // Declaration of class Fred
void someCode()
{
char memory[sizeof(Fred)]; // Line #1
void* place = memory; // Line #2
Fred* f = new(place) Fred(); // Line #3 (see "DANGER" below)
// The pointers f and place will be equal
...
}
Line #1 creates an array of sizeof(Fred) bytes of memory, which is big enough to hold a Fred
object. Line #2 creates a pointer place that points to the first byte of this memory (experienced C
programmers will note that this step was unnecessary; it's there only to make the code more
obvious). Line #3 essentially just calls the constructor Fred::Fred(). The this pointer in the Fred
constructor will be equal to place. The returned pointer f will therefore be equal to place.
ADVICE: Don't use this "placement new" syntax unless you have to. Use it only when you really
care that an object is placed at a particular location in memory. For example, when your
hardware has a memory-mapped I/O timer device, and you want to place a Clock object at that
memory location.
DANGER: You are taking sole responsibility that the pointer you pass to the "placement new"
operator points to a region of memory that is big enough and is properly aligned for the object
type that you're creating. Neither the compiler nor the run-time system make any attempt to
check whether you did this right. If your Fred class needs to be aligned on a 4 byte boundary but
you supplied a location that isn't properly aligned, you can have a serious disaster on your hands
(if you don't know what "alignment" means, please don't use the placement new syntax). You
have been warned.
You are also solely responsible for destructing the placed object. This is done by explicitly
calling the destructor:
void someCode()
{
char memory[sizeof(Fred)];
void* p = memory;
Fred* f = new(p) Fred();
...
f->~Fred(); // Explicitly call the destructor for the placed object
}
This is about the only time you ever explicitly call a destructor.
Note: there is a much cleaner but more sophisticated way of handling the destruction / deletion
situation.
Q: When I write a destructor, do I need to explicitly call the destructors for my member objects?
A: No. You never need to explicitly call a destructor (except with placement new).
A class's destructor (whether or not you explicitly define one) automagically invokes the
destructors for member objects. They are destroyed in the reverse order they appear within the
declaration for the class.
class Member {
public:
~Member();
...
};
class Fred {
public:
~Fred();
...
private:
Member x_;
Member y_;
Member z_;
};
Fred::~Fred()
{
// Compiler automagically calls z_.~Member()
// Compiler automagically calls y_.~Member()
// Compiler automagically calls x_.~Member()
}
Q: When I write a derived class's destructor, do I need to explicitly call the destructor for my
base class?
A: No. You never need to explicitly call a destructor (except with placement new).
A derived class's destructor (whether or not you explicitly define one) automagically invokes the
destructors for base class subobjects. Base classes are destructed after member objects. In the
event of multiple inheritance, direct base classes are destructed in the reverse order of their
appearance in the inheritance list.
class Member {
public:
~Member();
...
};
class Base {
public:
virtual ~Base(); // A virtual destructor
...
};
class Derived : public Base {
public:
~Derived();
...
private:
Member x_;
};
Derived::~Derived()
{
// Compiler automagically calls x_.~Member()
// Compiler automagically calls Base::~Base()
}
Note: Order dependencies with virtual inheritance are trickier. If you are relying on order
dependencies in a virtual inheritance hierarchy, you'll need a lot more information than is in this
FAQ.
Q: Is there any difference between List x; and List x();?
A: A big difference!
Suppose that List is the name of some class. Then function f() declares a local List object called
x:
void f()
{
List x; // Local object named x (of class List)
...
}
But function g() declares a function called x() that returns a List:
void g()
{
List x(); // Function named x (that returns a List)
...
}
Q: Can one constructor of a class call another constructor of the same class to initialize the this
object?
A: Nope.
Let's work an example. Suppose you want your constructor Foo::Foo(char) to call another
constructor of the same class, say Foo::Foo(char,int), in order that Foo::Foo(char,int) would help
initialize the this object. Unfortunately there's no way to do this in C++.
Some people do it anyway. Unfortunately it doesn't do what they want. For example, the line
Foo(x, 0); does not call Foo::Foo(char,int) on the this object. Instead it calls Foo::Foo(char,int) to
initialize a temporary, local object (not this), then it immediately destructs that temporary when
control flows over the ;.
class Foo {
public:
Foo(char x);
Foo(char x, int y);
...
};
Foo::Foo(char x)
{
...
Foo(x, 0); // this line does NOT help initialize the this object!!
...
}
You can sometimes combine two constructors via a default parameter:
class Foo {
public:
Foo(char x, int y=0); // this line combines the two constructors
...
};
If that doesn't work, e.g., if there isn't an appropriate default parameter that combines the two
constructors, sometimes you can share their common code in a private init() member function:
class Foo {
public:
Foo(char x);
Foo(char x, int y);
...
private:
void init(char x, int y);
};
Foo::Foo(char x)
{
init(x, int(x) + 7);
...
}
Foo::Foo(char x, int y)
{
init(x, y);
...
}
void Foo::init(char x, int y)
{
...
}
Q: Is the default constructor for Fred always Fred::Fred()?
A: No. A "default constructor" is a constructor that can be called with no arguments. One
example of this is a constructor that takes no parameters:
class Fred {
public:
Fred(); // Default constructor: can be called with no args
...
};
Another example of a "default constructor" is one that can take arguments, provided they are
given default values:
class Fred {
public:
Fred(int i=3, int j=5); // Default constructor: can be called with no args
...
};
Q: Should my constructors use "initialization lists" or "assignment"?
A: Initialization lists. In fact, constructors should initialize as a rule all member objects in the
initialization list. One exception is discussed further down.
Consider the following constructor that initializes member object x_ using an initialization list:
Fred::Fred() : x_(whatever) { }. The most common benefit of doing this is improved
performance. For example, if the expression whatever is the same type as member variable x_,
the result of the whatever expression is constructed directly inside x_ the compiler does not
make a separate copy of the object. Even if the types are not the same, the compiler is usually
able to do a better job with initialization lists than with assignments.
The other (inefficient) way to build constructors is via assignment, such as: Fred::Fred() { x_ =
whatever; }. In this case the expression whatever causes a separate, temporary object to be
created, and this temporary object is passed into the x_ object's assignment operator. Then that
temporary object is destructed at the ;. That's inefficient.
As if that wasn't bad enough, there's another source of inefficiency when using assignment in a
constructor: the member object will get fully constructed by its default constructor, and this
might, for example, allocate some default amount of memory or open some default file. All this
work could be for naught if the whatever expression and/or assignment operator causes the
object to close that file and/or release that memory (e.g., if the default constructor didn't allocate
a large enough pool of memory or if it opened the wrong file).
Conclusion: All other things being equal, your code will run faster if you use initialization lists
rather than assignment.
Note: There is no performance difference if the type of x_ is some built-in/intrinsic type, such as
int or char* or float. But even in these cases, my personal preference is to set those data members
in the initialization list rather than via assignment for consistency. Another symmetry argument
in favor of using initialization lists even for built-in/intrinsic types: non-static const and nonstatic
reference data members can't be assigned a value in the constructor, so for symmetry it
makes sense to initialize everything in the initialization list.
Now for the exceptions. Every rule has exceptions (hmmm; does "every rule has exceptions"
have exceptions? reminds me of Gdel's Incompleteness Theorems), and there are a couple of
exceptions to the "use initialization lists" rule. Bottom line is to use common sense: if it's
cheaper, better, faster, etc. to not use them, then by all means, don't use them. This might happen
when your class has two constructors that need to initialize the this object's data members in
different orders. Or it might happen when two data members are self-referential. Or when a datamember
needs a reference to the this object, and you want to avoid a compiler warning about
using the this keyword prior to the { that begins the constructor's body (when your particular
compiler happens to issue that particular warning). Or when you need to do an if/throw test on a
variable (parameter, global, etc.) prior to using that variable to initialize one of your this
members. This list is not exhaustive; please don't write me asking me to add another "Or
when...". The point is simply this: use common sense.
Q: Should you use the this pointer in the constructor?
A: Some people feel you should not use the this pointer in a constructor because the object is not
fully formed yet. However you can use this in the constructor (in the {body} and even in the
initialization list) if you are careful.
Here is something that always works: the {body} of a constructor (or a function called from the
constructor) can reliably access the data members declared in a base class and/or the data
members declared in the constructor's own class. This is because all those data members are
guaranteed to have been fully constructed by the time the constructor's {body} starts executing.
Here is something that never works: the {body} of a constructor (or a function called from the
constructor) cannot get down to a derived class by calling a virtual member function that is
overridden in the derived class. If your goal was to get to the overridden function in the derived
class, you won't get what you want. Note that you won't get to the override in the derived class
independent of how you call the virtual member function: explicitly using the this pointer (e.g.,
this->method()), implicitly using the this pointer (e.g., method()), or even calling some other
function that calls the virtual member function on your this object. The bottom line is this: even
if the caller is constructing an object of a derived class, during the constructor of the base class,
your object is not yet of that derived class. You have been warned.
Here is something that sometimes works: if you pass any of the data members in this object to
another data member's initializer, you must make sure that the other data member has already
been initialized. The good news is that you can determine whether the other data member has (or
has not) been initialized using some straightforward language rules that are independent of the
particular compiler you're using. The bad news it that you have to know those language rules
(e.g., base class sub-objects are initialized first (look up the order if you have multiple and/or
virtual inheritance!), then data members defined in the class are initialized in the order in which
they appear in the class declaration). If you don't know these rules, then don't pass any data
member from the this object (regardless of whether or not you explicitly use the this keyword) to
any other data member's initializer! And if you do know the rules, please be careful.
Q: What is the "Named Constructor Idiom"?
A: A technique that provides more intuitive and/or safer construction operations for users of your
class.
The problem is that constructors always have the same name as the class. Therefore the only way
to differentiate between the various constructors of a class is by the parameter list. But if there
are lots of constructors, the differences between them become somewhat subtle and error prone.
With the Named Constructor Idiom, you declare all the class's constructors in the private or
protected sections, and you provide public static methods that return an object. These static
methods are the so-called "Named Constructors." In general there is one such static method for
each different way to construct an object.
Q: What is virtual function?
A: When derived class overrides the base class method by redefining the same function, then if
client wants to access redefined the method from derived class through a pointer from base class
object, then you must define this function in base class as virtual function.
class parent
{
void Show()
{
cout << "i'm parent" << endl;
}
};
class child: public parent
{
void Show()
{
cout << "i'm child" << endl;
}
};
parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls parent->show()
now we goto virtual world...
class parent
{
virtual void Show()
{
cout << "i'm parent" << endl;
}
};
class child: public parent
{
void Show()
{
cout << "i'm child" << endl;
}
};
parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls child->show()
Q: What is a "pure virtual" member function?
A: The abstract class whose pure virtual method has to be implemented by all the classes which
derive on these. Otherwise it would result in a compilation error. This construct should be used
when one wants to ensure that all the derived classes implement the method defined as pure
virtual in base class.
Q: How virtual functions are implemented C++?
A: Virtual functions are implemented using a table of function pointers, called the vtable. There
is one entry in the table per virtual function in the class. This table is created by the constructor
of the class. When a derived class is constructed, its base class is constructed _rst which creates
the vtable. If the derived class overrides any of the base classes virtual functions, those entries in
the vtable are overwritten by the derived class constructor. This is why you should never call
virtual functions from a constructor: because the vtable entries for the object may not have
been set up by the derived class constructor yet, so you might end up calling base class
implementations of those virtual functions
Q: What is pure virtual function? or what is abstract class?
A: When you de_ne only function prototype in a base class without implementation and do the
complete implementation in derived class. This base class is called abstract class and client won't
able to instantiate an object using this base class. You can make a pure virtual function or
abstract class this way..
class Boo
{
void foo() = 0;
}
Boo MyBoo; // compilation error
Q: What is Pure Virtual Function? Why and when it is used?
A: The abstract class whose pure virtual method has to be implemented by all the classes which
derive on these. Otherwise it would result in a compilation error. This construct should be used
when one wants to ensure that all the derived classes implement the method defined as pure
virtual in base class.
Q: How Virtual functions call up is maintained?
A: Through Look up tables added by the compile to every class image. This also leads to
performance penalty.
Q: What is a virtual destructor?
A: The simple answer is that a virtual destructor is one that is declared with the virtual attribute.
The behavior of a virtual destructor is what is important. If you destroy an object through a caller
or reference to a base class, and the base-class destructor is not virtual, the derived-class
destructors are not executed, and the destruction might not be complete.
_ __ __ _ __ _ __ _
Q: What is inheritance?
A: Inheritance allows one class to reuse the state and behavior of another class. The derived class
inherits the properties and method implementations of the base class and extends it by overriding
methods and adding additional properties and methods.
Q: When should you use multiple inheritance?
A:There are three acceptable answers:- "Never,""Rarely," and "When the problem domain
cannot be accurately modeled any other way." Consider an Asset class, Building class, Vehicle
class, and CompanyCar class. All company cars are vehicles. Some company cars are assets
because the organizations own them. Others might be leased. Not all assets are vehicles. Money
accounts are assets. Real estate holdings are assets. Some real estate holdings are buildings. Not
all buildings are assets. Ad infinitum. When you diagram these relationships, it becomes
apparent that multiple inheritance is a likely and intuitive way to model this common problem
domain. The applicant should understand, however, that multiple inheritance, like a chainsaw, is
a useful tool that has its perils, needs respect, and is best avoided except when nothing else will
do.
Q: Explain the ISA and HASA class relationships. How would you implement each in a class
design?
A: A specialized class "is" a specialization of another class and, therefore, has the ISA
relationship with the other class. This relationship is best implemented by embedding an object
of the Salary class in the Employee class.
Q: When is a template a better solution than a base class?
A: When you are designing a generic class to contain or otherwise manage objects of other
types, when the format and behavior of those other types are unimportant to their containment or
management, and particularly when those other types are unknown (thus, the generality) to the
designer of the container or manager class.
Q: What is multiple inheritance(virtual inheritance)? What are its advantages and disadvantages?
A: Multiple Inheritance is the process whereby a child can be derived from more than one parent
class. The advantage of multiple inheritance is that it allows a class to inherit the functionality of
more than one base class thus allowing for modeling of complex relationships.
The disadvantage of multiple inheritance is that it can lead to a lot of confusion(ambiguity)
when two base classes implement a method with the same name.
Q: What a derived class inherits or doesn't inherit?
A: Inherits:
Every data member defined in the parent class (although such members may not always be
accessible in the derived class!)
Every ordinary member function of the parent class (although such members may not always be
accessible in the derived class!)
The same initial data layout as the base class.
Doesn't Inherit :
The base class's constructors and destructor.
The base class's assignment operator.
The base class's friends
_ __ _ _ __ _ _ _ __ _
Q: What is Polymorphism??
A: Polymorphism allows a client to treat di_erent objects in the same way even if they were
created from di_erent classes and exhibit di_erent behaviors. You can use implementation
inheritance to achieve polymorphism in languages such as C++ and Java. Base class object's
pointer can invoke methods in derived class objects. You can also achieve polymorphism in C++
by function overloading and operator overloading.
Q: What is problem with Runtime type identification?
A: The run time type identification comes at a cost of performance penalty. Compiler maintains
the class.
Q: What is Polymorphism?
A: Polymorphism allows a client to treat different objects in the same way even if they were
created from different classes and exhibit different behaviors. You can use implementation
inheritance to achieve polymorphism in languages such as C++ and Java. Base class object's
pointer can invoke methods in derived class objects. You can also achieve polymorphism in
C++ by function overloading and operator overloading.
__ _ _____
Q: What is a class?
A: A class is an expanded concept of a data structure: instead of holding only data, it can hold
both data and functions.
Q: What are the differences between a C++ struct and C++ class?
A: The default member and base class access specifies are different. This is one of the
commonly misunderstood aspects of C++. Believe it or not, many programmers think that a C++
struct is just like a C struct, while a C++ class has inheritance, access specifes, member
functions, overloaded operators, and so on. Actually, the C++ struct has all the features of the
class. The only differences are that a struct defaults to public member access and public base
class inheritance, and a class defaults to the private access specified and private base-class
inheritance.
Q: How do you know that your class needs a virtual destructor?
A: If your class has at least one virtual function, you should make a destructor for this class
virtual. This will allow you to delete a dynamic object through a caller to a base class object. If
the destructor is non-virtual, then wrong destructor will be invoked during deletion of the
dynamic object.
Q: What is encapsulation?
A: Containing and hiding Information about an object, such as internal data structures and code.
Encapsulation isolates the internal complexity of an object's operation from the rest of the
application. For example, a client component asking for net revenue from a business object need
not know the data's origin.
Q: What is "this" pointer?
A: The this pointer is a pointer accessible only within the member functions of a class, struct, or
union type. It points to the object for which the member function is called. Static member
functions do not have a this pointer. When a nonstatic member function is called for an
object, the address of the object is passed as a hidden argument to the function. For example, the
following function call
myDate.setMonth( 3 );
can be interpreted this way:
setMonth( &myDate, 3 );
The object's address is available from within the member function as the this pointer. It is legal,
though unnecessary, to use the this pointer when referring to members of the class.
Q: What happens when you make call "delete this;"?
A: The code has two built-in pitfalls. First, if it executes in a member function for an extern,
static, or automatic object, the program will probably crash as soon as the delete statement
executes. There is no portable way for an object to tell that it was instantiated on the heap, so the
class cannot assert that its object is properly instantiated. Second, when an object commits
suicide this way, the using program might not know about its demise. As far as the instantiating
program is concerned, the object remains in scope and continues to exist even though the object
did itself in. Subsequent dereferencing of the pointer can and usually does lead to disaster.
You should never do this. Since compiler does not know whether the object was allocated on the
stack or on the heap, "delete this" could cause a disaster.
Q: What is assignment operator?
A: Default assignment operator handles assigning one object to another of the same class.
Member to member copy (shallow copy)
Q: What are all the implicit member functions of the class? Or what are all the functions which
compiler implements for us if we don't define one?
A:
(a) default ctor
(b) copy ctor
(c) assignment operator
(d) default destructor
(e) address operator
Q: What is a container class? What are the types of container classes?
A: A container class is a class that is used to hold objects in memory or external storage. A
container class acts as a generic holder. A container class has a predefined behavior and a wellknown
interface. A container class is a supporting class whose purpose is to hide the topology
used for maintaining the list of objects in memory. When a container class contains a group of
mixed objects, the container is called a heterogeneous container; when the container is holding a
group of objects that are all the same, the container is called a homogeneous container.
Q: What is Overriding?
A: To override a method, a subclass of the class that originally declared the method must declare
a method with the same name, return type (or a subclass of that return type), and same parameter
list.
The definition of the method overriding is:
· Must have same method name.
· Must have same data type.
· Must have same argument list.
Overriding a method means that replacing a method functionality in child class. To imply
overriding functionality we need parent and child classes. In the child class you define the same
method signature as one defined in the parent class.
Q: What is a nested class? Why can it be useful?
A: A nested class is a class enclosed within the scope of another class. For example:
// Example 1: Nested class
//
class OuterClass
{
class NestedClass
{
// ...
};
// ...
};
Nested classes are useful for organizing code and controlling access and dependencies. Nested
classes obey access rules just like other parts of a class do; so, in Example 1, if NestedClass is
public then any code can name it as OuterClass::NestedClass. Often nested classes contain
private implementation details, and are therefore made private; in Example 1, if NestedClass
is private, then only OuterClass's members and friends can use NestedClass. When you
instantiate as outer class, it won't instantiate inside class.
Q: What is a local class? Why can it be useful?
A: Local class is a class defined within the scope of a function _ any function, whether a
member function or a free function. For example:
// Example 2: Local class
//
int f()
{
class LocalClass
{
// ...
};
// ...
};
Like nested classes, local classes can be a useful tool for managing code dependencies.
Q: What a derived class can add?
A: New data members
New member functions
New constructors and destructor
New friends
Q: What happens when a derived-class object is created and destroyed?
A: Space is allocated (on the stack or the heap) for the full object (that is, enough space to store
the data members inherited from the base class plus the data members defined in the derived
class itself)
The base class's constructor is called to initialize the data members inherited from the base class
The derived class's constructor is then called to initialize the data members added in the derived
class
The derived-class object is then usable
When the object is destroyed (goes out of scope or is deleted) the derived class's destructor is
called on the object first
Then the base class's destructor is called on the object
Finally the allocated space for the full object is reclaimed
Q: What is the difference between new/delete and malloc/free?
A: Malloc/free do not know about constructors and destructors. New and delete create and
destroy objects, while malloc and free allocate and deallocate memory.
Q:What is difference between new and malloc?
A: Both malloc and new functions are used for dynamic memory allocations and the basic
difference is: malloc requires a special "typecasting" when it allocates memory for eg. if the
pointer used is the char pointer then after the processor allocates memory then this allocated
memory needs to be typecasted to char pointer i.e (char*).but new does not requires any
typecasting. Also, free is the keyword used to free the memory while using malloc and delete the
keyword to free memory while using new, otherwise this will lead the memory leak.
Q: What is the difference between delete and delete[]?
A: Whenever you allocate memory with new[], you have to free the memory using delete[].
When you allocate memory with 'new', then use 'delete' without the brackets. You use new[] to
allocate an array of values (always starting at the index 0).
Q: What is difference between malloc()/free() and new/delete?
A: malloc allocates memory for object in heap but doesn't invoke object's constructor to initialize
the object. new allocates memory and also invokes constructor to initialize the object. malloc()
and free() do not support object semantics, does not construct and destruct objects
Q: What is the difference between "new" and "operator new" ?
A:"operator new" works like malloc.
Q: What is Memory alignment??
A: The term alignment primarily means the tendency of an address pointer value to be a multiple
of some power of two. So a pointer with two byte alignment has a zero in the least signi_cant bit.
And a pointer with four byte alignment has a zero in both the two least signi_cant bits. And so
on. More alignment means a longer sequence of zero bits in the lowest bits of a pointer.
Q: How does free know the size of memory to be deleted.?
int *i = (int *)malloc(12); followed by free(i); how did free function call know how much of
memory to delete?
A: It depends on the implementation, but there is usually a malloc header added to all the
memory allocated through malloc. on Linux its 4 bytes of memory preceding the memory
returned to you, whihc contains the number of bytes allocated + 4(itself).
so when you say,
int *i = (int *)malloc(12);
it allocates 16 bytes.
As you can see above total of 16 bytes are allocated, first 4 bytes stores the number of bytes
allocated(offset to the next memory from the start of this memory).
address of the 5th byte is returned to i. now i can access 12 bytes from this byte.
Q: Can I free() pointers allocated with new? Can I delete pointers allocated with malloc()?
A: No!
It is perfectly legal, moral, and wholesome to use malloc() and delete in the same program, or to
use new and free() in the same program. But it is illegal, immoral, and despicable to call free()
with a pointer allocated via new, or to call delete on a pointer allocated via malloc().
Beware! I occasionally get e-mail from people telling me that it works OK for them on machine
X and compiler Y. Just because they don't see bad symptoms in a simple test case doesn't mean it
won't crash in the field. Even if they know it won't crash on their particular compiler doesn't
mean it will work safely on another compiler, another platform, or even another version of the
same compiler.
Q: Why should I use new instead of trustworthy old malloc()?
A: Constructors/destructors, type safety, overridability.
Constructors/destructors: unlike malloc(sizeof(Fred)), new Fred() calls Fred's constructor.
Similarly, delete p calls *p's destructor.
Type safety: malloc() returns a void* which isn't type safe. new Fred() returns a pointer of the
right type (a Fred*).
Overridability: new is an operator that can be overridden by a class, while malloc() is not
overridable on a per-class basis.
Q: Can I use realloc() on pointers allocated via new?
A: No!
When realloc() has to copy the allocation, it uses a bitwise copy operation, which will tear many
C++ objects to shreds. C++ objects should be allowed to copy themselves. They use their own
copy constructor or assignment operator.
Besides all that, the heap that new uses may not be the same as the heap that malloc() and
realloc() use!
Q: What are C++ storage classes?
A:
auto: the default. Variables are automatically created and initialized when they are defined and
are destroyed at the end of the block containing their definition. They are not visible
outside that block.
register: a type of auto variable. a suggestion to the compiler to use a CPU register for
performance.
static: a variable that is known only in the function that contains its definition but is never
destroyed and retains its value between calls to that function. It exists from the time the
program begins execution.
extern: a static variable whose definition and placement is determined when all object and library
modules are combined (linked) to form the executable code file. It can be visible outside
the file where it is defined.
Q: What are storage qualifiers in C++ ?
A: They are:
Const: Indicates that memory once initialized, should not be altered by a program.
Volatile: Indicates that the value in the memory location can be altered even though nothing
in the program code modifies the contents. for example if you have a pointer to hardware
location that contains the time, where hardware changes the value of this pointer variable
and not the program. The intent of this keyword to improve the optimization ability of the
compiler.
Mutable: Iindicates that particular member of a structure or class can be altered even if a
particular structure variable, class, or class member function is constant.
Q: What is reference ??
A: reference is a name that acts as an alias, or alternative name, for a previously defined variable
or an object. prepending variable with "&" symbol makes it as reference.
for example:
int a;
int &b = a;
Q: What is passing by reference?
A: Method of passing arguments to a function which takes parameter of type reference.
for example:
void swap( int & x, int &amp;amp;amp; y )
{
int temp = x;
x = y;
y = temp;
}
int a=2, b=3;
swap( a, b );
Basically, inside the function there won't be any copy of the arguments "x" and "y" instead they
refer to original variables a and b. so no extra memory needed to pass arguments and it is more
efficient.
Q: When do use "const" reference arguments in function?
A:
a) Using const protects you against programming errors that inadvertently alter data.
b) Using const allows function to process both const and non-const actual arguments, while a
function without const in the prototype can only accept non constant arguments.
c) Using a const reference allows the function to generate and use a temporary variable
appropriately.
Q: When are temporary variables created by C++ compiler?
A: Provided that function parameter is a "const reference", compiler generates temporary
variable
in following2 ways.
a) The actual argument is the correct type, but it
isn't Lvalue
double Cube(const double & num)
{
num = num * num * num;
return num;
}
double temp = 2.0;
double value = cube(3.0 + temp); // argument is a expression and not a Lvalue;
b) The actual argument is of the wrong type, but of a type that can be converted to the correct
type
long temp = 3L;
double value = cuberoot ( temp); // long to double conversion
Q: What problem does the namespace feature solve?
A: Multiple providers of libraries might use common global identifiers causing a name collision
when an application tries to link with two or more such libraries. The namespace feature
surrounds a library's external declarations with a unique namespace that eliminates the potential
for those collisions.
namespace [identifier] { namespace-body }
A namespace declaration identifies and assigns a name to a declarative region.
The identifier in a namespace declaration must be unique in the declarative region in which it is
used. The identifier is the name of the namespace and is used to reference its members.
Q: What is the use of 'using' declaration?
A: A using declaration makes it possible to use a name from a namespace without the scope
operator.
Q: What is an Iterator class?
A: A class that is used to traverse through the objects maintained by a container class. There are
_ve categories of iterators: input iterators, output iterators, forward iterators, bidirectional
iterators, random access. An iterator is an entity that gives access to the contents of a container
object without violating encapsulation constraints. Access to the contents is granted on a one-atatime
basis in order. The order can be storage order (as in lists and queues) or some arbitrary
order (as in array indices) or according to some ordering relation (as in an ordered binary tree).
The iterator is a construct, which provides an interface that, when called, yields either the next
element in the container, or some value denoting the fact that there are no more elements to
examine. Iterators hide the details of access to and update of the elements of a container class.
Something like a pointer.
Q: What do you mean by Stack unwinding?
A: It is a process during exception handling when the destructor is called for all local objects in
the stack between the place where the exception was thrown and where it is caught.
Q: Implementation of string length using pointer hopping
A: #include
// Test to see if pointer hopping is worthwhile.
Q: What is inline function?
A: The inline keyword tells the compiler to substitute the code within the function de_nition for
every instance of a function call. However, substitution occurs only at the compiler's discretion.
For example, the compiler does not inline a function if its address is taken or if it is too large to
inline.
Q: What is name mangling in C++??
A: The process of encoding the parameter types with the function/method name into a unique
name is called name mangling. The inverse process is called demangling.
For example Foo::bar(int, long) const is mangled as 'bar__C3Fooil'.
For a constructor, the method name is left out.
That is Foo::Foo(int, long) const is mangled as „C3Fooil?
Q: Can you think of a situation where your program would crash without reaching the
breakpoint which you set at the beginning of main()?
A: C++ allows for dynamic initialization of global variables before main() is invoked. It is possible that initialization of global will invoke some function. If this function crashes the crash
will occur before main() is entered. Name two cases where you MUST use initialization
list as opposed to assignment in constructors. Both non-static const data members and reference
data members cannot be assigned values; instead, you should use initialization list to initialize
them.
Q: What is faster ++i or i++, where i is an interger variable?
A: The answer to this lies in the fact, how they used. With ++i(PreIncrement) the variable is
incremented and new value is returned. So it requires one instruction to increment the variable.
In case of i++(post Increment), the old value has to be returned or used in the expression and
then the variable is incrememented after the expression is evaluated. Since you need one
instruction to save the old value to be used in the expression and other instruction to increment
the variable, its comparatively slower.
0 comments:
Post a Comment
Thanks For Your Comment. We Get Back To You As Soon As Possible.