C++ Object Interviews Questions II

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. What are proxy objects? Objects that stand for other objects are called proxy objects or surrogates.
   

   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.
8. Name some pure object oriented languages. Smalltalk, Java, Eiffel, Sather.
9. Name the operators that cannot be overloaded. sizeof, ., .*, .->, ::, ?: Salam in the comments notes that -> can be overloaded.
10. 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.
11. What is an orthogonal base class?
    If two base classes have no overlapping methods or data they are said
    to be independent of, or orthogonal to each other. Orthogonal in the
    sense means that two classes operate in different dimensions and do not
    interfere with each other in any way. The same derived class may
    inherit such classes with no difficulty.
12. 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.