C++ Interview Questions and Answers Part(1)

Check out this collection of C++ Interview Questions and Answers.This is the new series on C++ Interview Questions and Answers from very basic level to expert level.This is the first episode of a series.

What is C++?
What is a modifier?
What is an accessor?
Differentiate between a template class and class template.
When does a name clash occur?
Define namespace.
What is the use of ‘using’ declaration.
What is an Iterator class?
What is an incomplete type?
What is a dangling pointer?
Differentiate between the message and method.
What is an adaptor class or Wrapper class?
What is a Null object?
What is class invariant?
What do you mean by Stack unwinding?
Define precondition and post-condition to a member function.
What are the conditions that have to be met for a condition to be an invariant of the class?
What are proxy objects?
Name some pure object oriented languages.
What is a node class?

What is C++?

Released in 1985, C++ is an object-oriented programming language created by Bjarne Stroustrup. C++ maintains almost all aspects of the C language, while simplifying memory management and adding several features - including a new datatype known as a class (you will learn more about these later) - to allow object-oriented programming. C++ maintains the features of C which allowed for low-level memory access but also gives the programmer new tools to simplify memory management.

C++ used for:
C++ is a powerful general-purpose programming language. It can be used to create small programs or large applications. It can be used to make CGI scripts or console-only DOS programs. C++ allows you to create programs to do almost anything you need to do. The creator of C++, Bjarne Stroustrup, has put together a partial list of applications written in C++.

What is a modifier?

A modifier, also called a modifying 
function is a member function that
changes the value of at least one 
data member. In other words, an
operation that modifies the state
of an object. Modifiers are also 
known as ‘mutators’. Example: 
The function mod is a modifier in the
      following code snippet:

          class test
          {
              int x,y;
              public:
               test()
               {
                  x=0; y=0;
               }
           void mod()
               {
                  x=10;
                  y=15;
               }
          };

What is an accessor?

An accessor is a class operation that does not modify the state of an object. The accessor functions need to be declared as const operations

Differentiate between a template class and class template.

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.

When does a name clash occur?

A name clash occurs when a name is defined in more than one place. For example., two different class libraries could give two different classes the same name. If you try to use many class libraries at the same time, there is a fair chance that you will be unable to compile or link the program because of name clashes.

Define namespace.

It is a feature in C++ to minimize name collisions in the global name space. This namespace keyword assigns a distinct name to a library that allows other libraries to use the same identifier names without creating any name collisions. Furthermore, the compiler uses the namespace signature for differentiating the definitions.

What is the use of ‘using’ declaration.

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

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.
The simplest and safest iterators are those that permit read-only access to the contents of a container class.

What is an incomplete type?

Incomplete types 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.

int *i=0x400 // i points to address 400
*i=0; //set the value of memory location pointed by i.

Incomplete types are otherwise called uninitialized pointers.

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. The following
code snippet shows this:

          class Sample
          {
          public:
                  int *ptr;
                  Sample(int i)
                  {
                   ptr = new int(i);
                  }

                  ~Sample()
                  {
                   delete ptr;
                  }
                  void PrintVal()
                  {
cout << "The value is " << *ptr;
                  }
          };

          void SomeFunc(Sample x)
          {
cout << "Say i am in someFunc " << endl;
          }

          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.

Differentiate between the message and method.
Message:
* Objects communicate by sending messages to each other.
* A message is sent to invoke a method.

Method
* Provides response to a message.
* It is an implementation of an operation.
What is an adaptor class or Wrapper class?
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.
 What is a Null object?
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.
What is class invariant?
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.
What do you mean by Stack unwinding?
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.
 Define precondition and post-condition to a member function.
Precondition: A precondition is a condition that must be true on entry to a member function. A class is used correctly if preconditions are never false. An operation is not responsible for doing anything sensible if its precondition fails to hold. For example, the interface invariants of stack class say nothing about pushing yet another element on a stack that is already full. We say that isful() is a precondition of the push operation. Post-condition: A post-condition is a condition that must be true on exit from a member function if the precondition was valid on entry to that function. A class is implemented correctly if post-conditions are never false. For example, after pushing an element on the stack, we know that isempty() must necessarily hold. This is a post-condition of the push operation.
 What are the conditions that have to be met for a condition to be an invariant of the class?
* The condition should hold at the end of every constructor.
* The condition should hold at the end of every mutator (non-const) operation.
 What are proxy objects?
Objects that stand for other objects are called proxy objects or surrogates. 
template 
class Array2D
{
public:
class Array1D
{
public:
T& operator[] (int index);
const T& operator[] (int index)const;
};

Array1D operator[] (int index);
const Array1D operator[] (int index) const;
};

The following then becomes legal:

Array2Ddata(10,20);
cout<

Here data[3] yields an Array1D object and the operator [] invocation on that object yields the float in position(3,6) of the original two dimensional array. Clients of the Array2D class need not be aware of the presence of the Array1D class. Objects of this latter class stand for one-dimensional array objects that, conceptually, do not exist for clients of Array2D. Such clients program as if they were using real, live, two-dimensional arrays. Each Array1D object stands for a one-dimensional array that is absent from a conceptual model used by the clients of Array2D. In the above example, Array1D is a proxy class. Its instances stand for one-dimensional arrays that, conceptually, do not exist.
Name some pure object oriented languages.
Smalltalk, Java, Eiffel, Sather.
 What is a node class?
A node class is a class that,
* relies on the base class for services and implementation,
* provides a wider interface to the users than its base class,
* relies primarily on virtual functions in its public interface
* depends on all its direct and indirect base class
* can be understood only in the context of the base class
* can be used as base for further derivation
* can be used to create objects.
A node class is a class that has added new services or functionality beyond the services inherited from its base class.