C++ Aptitude and OOPS page 1, 2, 3, 4, 5

16.   What is class invariant?
Answer:
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.

17.   What do you mean by Stack unwinding?
Answer:
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.
              
18.   Define precondition and post-condition to a member function.
Answer:
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.

19.   What are the conditions that have to be met for a condition to be an invariant of the class?
Answer:

1. The condition should hold at the end of every constructor.
2. The condition should hold at the end of every mutator(non-const) operation.

     
20.   What are proxy objects?
Answer:
            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.
      
21.   Name some pure object oriented languages.
Answer:

1.   Smalltalk,
2.   Java,
3.   Eiffel, 
4.   Sather.

22.   Name the operators that cannot be overloaded.   
Answer:
sizeof    .          .*         .->       ::           ?:                     

23.   What is a node class?
Answer:
A node class is a class that,

1. relies on the base class for services and implementation,
2. provides a wider interface to te users than its base class,
3. relies primarily on virtual functions in its public interface
4. depends on all its direct and indirect base class
5. can be understood only in the context of the base class
6. can be used as base for further derivation
7. 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.   

24.   What is an orthogonal base class?
Answer:
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.

25. What is a container class? What are the types of container classes?
Answer:
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.

26. What is a protocol class?
Answer:
An abstract class is a protocol class if:

1. it neither contains nor inherits from classes that contain member data, non-virtual functions, or private (or protected) members of any kind.
2. it has a non-inline virtual destructor defined with an empty implementation,
3. all member functions other than the destructor including inherited functions, are declared pure virtual functions and left undefined.

27. What is a mixin class?
Answer:
A class that provides some but not all of the implementation for a virtual base class is often called mixin. Derivation done just for the purpose of redefining the virtual functions in the base classes is often called mixin inheritance. Mixin classes typically don't share common bases.

28. What is a concrete class?
Answer:
A concrete class is used to define a useful object that can be instantiated as an automatic variable on the program stack. The implementation of a concrete class is defined. The concrete class is not intended to be a base class and no attempt to minimize dependency on other classes in the implementation or behavior of the class.

29.What is the handle class?
Answer:
A handle is a class that maintains a pointer to an object that is programmatically accessible through the public interface of the handle class.
Explanation:
In case of abstract classes, unless one manipulates the objects of these classes through pointers and references, the benefits of the virtual functions are lost. User code may become dependent on details of implementation classes because an abstract type cannot be allocated statistically or on the stack without its size being known. Using pointers or references implies that the burden of memory management falls on the user. Another limitation of abstract class object is of fixed size. Classes however are used to represent concepts that require varying amounts of storage to implement them.
A popular technique for dealing with these issues is to separate what is used as a single object in two parts: a handle providing the user interface and a representation holding all or most of the object's state. The connection between the handle and the representation is typically a pointer in the handle. Often, handles have a bit more data than the simple representation pointer, but not much more. Hence the layout of the handle is typically stable, even when the representation changes and also that handles are small enough to move around relatively freely so that the user needn’t use the pointers and the references.   
  
 30. What is an action class?
Answer:
The simplest and most obvious way to specify an action in C++ is to write a function. However, if the action has to be delayed, has to be transmitted 'elsewhere' before being performed, requires its own data, has to be combined with other actions, etc then it often becomes attractive to provide the action in the form of a class that can execute the desired action and provide other services as well. Manipulators used with iostreams is an obvious example.
Explanation:
            A common form of action class is a simple class containing just one virtual function.
         class Action
       {
               public:
                    virtual int do_it( int )=0;
                    virtual ~Action( );
         }
Given this, we can write code say a member that can store actions for later execution without using pointers to functions, without knowing anything about the objects involved, and without even knowing the name of the operation it invokes. For example:
class write_file : public Action
     {
              File& f;
              public:
                  int do_it(int)
                 {
                       return fwrite( ).suceed( );
                 }
      };
     class error_message: public Action
     {
                response_box db(message.cstr( ),"Continue","Cancel","Retry");
                switch (db.getresponse( ))
                {
                        case 0: return 0;
                        case 1: abort();
                        case 2: current_operation.redo( );return 1;
                 }
      };  

A user of the Action class will be completely isolated from any knowledge of derived classes such as write_file and error_message.

C++ Aptitude and OOPS page 1, 2, 3, 4, 5