C/C++ Interview Questions : Base Cleaning

1. 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.
Example:

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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. Define pre-condition and post-condition to a member function.
Pre-condition:
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.

9. 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.

10. What are proxy objects?
Objects that stand for other objects are called proxy objects or surrogates. Example:

template<class T>
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:
Array2D<float>data(10,20);
........
cout<<data[3][6];     //  fine

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.