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C++ Interview Questions

                                           C++ Freaquently asked Questions

1) What is encapsulation??

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.

2) What is inheritance?

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.

3) What is Polymorphism??

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.

4) What is constructor or ctor?

Constructor creates an object and initializes it. It also creates vtable for virtual

functions. It is different from other methods in a class.

 5) What is destructor?

Destructor usually deletes any extra resources allocated by the object.

6) What is copy constructor?

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:

Boo Obj1(10); // calling Boo constructor

Boo Obj2(Obj1); // calling boo copy constructor

Boo Obj2 = Obj1;// calling boo copy constructor

 7) When are copy constructors called?

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

 8) What is assignment operator?

Default assignment operator handles assigning one object to another of the same class.

Member to member copy (shallow copy)

 9) 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.??

default ctor

copy ctor

assignment operator

default destructor

address operator

 10) What is conversion constructor?

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

 11) What is conversion operator??

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.

 12) What is diff between malloc()/free() and new/delete?

malloc allocates memory for object in heap but doesn t invoke object s constructor to

initiallize 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

string * ptr = (string *)(malloc (sizeof(string)))

Are not safe

Does not calculate the size of the objects that it construct

Returns a pointer to void

int *p = (int *) (malloc(sizeof(int)));

int *p = new int;

Are not extensible

new and delete can be overloaded in a class

"delete" first calls the object s termination routine (i.e. its destructor) and then

releases the space the object occupied on the heap memory. If an array of objects was

created using new, then delete must be told that it is dealing with an array by preceding

the name with an empty []:-

Int_t *my_ints = new Int_t[10];

...

delete []my_ints;

13) What is the diff between "new" and "operator new" ?

"operator new" works like malloc.

14) What is difference between template and macro??

There is no way for the compiler to verify that the macro parameters are of compatible

types. The macro is expanded without any special type checking.

If macro parameter has a postincremented variable ( like c++ ), the increment is performed

two times.

Because macros are expanded by the preprocessor, compiler error messages will refer to the

expanded macro, rather than the macro definition itself. Also, the macro will show up in

expanded form during debugging.

for example:

Macro:

#define min(i, j) (i < j ? i : j)

template:

template

T min (T i, T j)

{

return i < j ? i : j;

}

15) What are C++ storage classes?

auto

register

static

extern

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.

16) What are storage qualifiers in C++ ?

They are..

const

volatile

mutable

Const keyword indicates that memory once initialized, should not be altered by a program.

volatile keyword 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 keyword indicates that particular member of a structure or class can be altered even

if a particular structure variable, class, or class member function is constant.

struct data

{

char name[80];

mutable double salary;

}

const data MyStruct = { "Satish Shetty", 1000 }; //initlized by complier

strcpy ( MyStruct.name, "Shilpa Shetty"); // compiler error

MyStruct.salaray = 2000 ; // complier is happy allowed

17) What is reference ??

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;

18) What is passing by reference?

Method of passing arguments to a function which takes parameter of type reference. for

example:

void swap( int & x, int & y )

{

int temp = x;

x = y;

y = x;

}

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.

19) When do use "const" reference arguments in function?

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.

20) When are temporary variables created by C++ compiler?

Provided that function parameter is a "const reference", compiler generates temporary

variable in following 2 ways.

a) The actual argument is the correct type, but it isn t Lvalue

double Cuberoot ( const double & num )

{

num = num * num * num;

return num;

}

double temp = 2.0;

double value = cuberoot ( 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

21) What is virtual function?

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() i

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()

22) What is pure virtual function? or what is abstract class?

When you define only function prototype in a base class without 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

23) What is Memory alignment??

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

significant bit. And a pointer with four byte alignment has a zero in both the two least

significant bits. And so on. More alignment means a longer sequence of zero bits in the

lowest bits of a pointer.

24) What problem does the namespace feature solve?

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.

25) What is the use of  using  declaration?

A using declaration makes it possible to use a name from a namespace without the scope

operator.

26) What is an Iterator class?

A class that is used to traverse through the objects maintained by a container class. There

are five 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-at-a-time 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.

27) What is a dangling pointer?

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.

What do you mean by Stack unwinding?

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.

28) Name the operators that cannot be overloaded??

sizeof, ., .*, .->, ::, ?:

What is a container class? What are the types of container classes?

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.

29) What is inline function??

The __inline keyword tells the compiler to substitute the code within the function

definition 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.

30) What is overloading??

With the C++ language, you can overload functions and operators. Overloading is the practice

of supplying more than one definition for a given function name in the same scope.

- Any two functions in a set of overloaded functions must have different argument lists.

- Overloading functions with argument lists of the same types, based on return type alone,

is an error.

31) What is Overriding?

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.

32) What is "this" pointer?

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.

33) What happens when you make call "delete this;" ??

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.

34) How virtual functions are implemented C++?

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 first 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

35) What is name mangling in C++??

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 .

What is the difference between a pointer and a reference?

A reference must always refer to some object and, therefore, must always be initialized;

pointers do not have such restrictions. A pointer can be reassigned to point to different

objects while a reference always refers to an object with which it was initialized.

How are prefix and postfix versions of operator++() differentiated?

The postfix version of operator++() has a dummy parameter of type int. The prefix version

does not have dummy parameter.

What is the difference between const char *myPointer and char *const myPointer?

Const char *myPointer is a non constant pointer to constant data; while char *const

myPointer is a constant pointer to non constant data.

How can I handle a constructor that fails?

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.

How can I handle a destructor that fails?

Write a message to a log-file. 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 (Bar e) {

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.

36) What is Virtual Destructor?

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.

Can you think of a situation where your program would crash without reaching the breakpoint

37) which you set at the beginning of main()?

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.

38) Can you overload a function based only on whether a parameter is a value or a reference?

No. Passing by value and by reference looks identical to the caller.

39) What are the differences between a C++ struct and C++ class?

The default member and base class access specifiers are different.

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 specifier and private base class inheritance.

40) What does extern "C" int func(int *, Foo) accomplish?

It will turn off "name mangling" for func so that one can link to code compiled by a C

compiler.

41) How do you access the static member of a class?

::

42) What is multiple inheritance(virtual inheritance)? What are its advantages and

disadvantages?

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.

43) What are the access privileges in C++? What is the default access level?

The access privileges in C++ are private, public and protected. The default access level

assigned to members of a class is private. Private members of a class are accessible only

within the class and by friends of the class. Protected members are accessible by the class

itself and it s sub-classes. Public members of a class can be accessed by anyone.

44) What is a nested class? Why can it be useful?

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.

45) What is a local class? Why can it be useful?

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.

46) Can a copy constructor accept an object of the same class as parameter, instead of reference of the object?

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.

(From Microsoft) Assume I have a linked list contains all of the alphabets from  A  to  Z .

47) I want to find the letter  Q  in the list, how does you perform the search to find the  Q ?

How do you write a function that can reverse a linked-list? (Cisco System)

void reverselist(void)

{

if(head==0)

return;

if(head->next==0)

return;

if(head->next==tail)

{

head->next = 0;

tail->next = head;

}

else

{

node* pre = head;

node* cur = head->next;

node* curnext = cur->next;

head->next = 0;

cur->next = head;

for(; curnext!=0; )

{

cur->next = pre;

pre = cur;

cur = curnext;

curnext = curnext->next;

}

curnext->next = cur;

}

}

48) How do you find out if a linked-list has an end? (i.e. the list is not a cycle)

You can find out by using 2 pointers. One of them goes 2 nodes each time. The second one

goes at 1 nodes each time. If there is a cycle, the one that goes 2 nodes each time will

eventually meet the one that goes slower. If that is the case, then you will know the

linked-list is a cycle.

49) How can you tell what shell you are running on UNIX system?

You can do the Echo $RANDOM. It will return a undefined variable if you are from the

C-Shell, just a return prompt if you are from the Bourne shell, and a 5 digit random numbers

if you are from the Korn shell. You could also do a ps -l and look for the shell with the

highest PID.

50) What is Boyce Codd Normal form?

A relation schema R is in BCNF with respect to a set F of functional dependencies if for all

functional dependencies in F+ of the form a->b, where a and b is a subset of R, at least one

of the following holds:

• a->b is a trivial functional dependency (b is a subset of a)

• a is a superkey for schema R

51) Could you tell something about the Unix System Kernel?

The kernel is the heart of the UNIX openrating system, it s reponsible for controlling the

computer s resouces and scheduling user jobs so that each one gets its fair share of

resources.

52) What is a Make file?

Make file is a utility in Unix to help compile large programs. It helps by only compiling

the portion of the program that has been changed

53) How do you link a C++ program to C functions?

By using the extern "C" linkage specification around the C function declarations.

Explain the scope resolution operator.

Design and implement a String class that satisfies the following:

Supports embedded nulls

Provide the following methods (at least)

Constructor

Destructor

Copy constructor

Assignment operator

Addition operator (concatenation)

Return character at location

Return substring at location

Find substring

Provide versions of methods for String and for char* arguments

Suppose that data is an array of 1000 integers. Write a single function call that will sort

the 100 elements data [222] through data [321].

Answer: quicksort ((data + 222), 100);

54) What is default constructor?

Constructor with no arguments or all the arguments has default values.