Page 229 - DCAP404 _Object Oriented Programming

Object-oriented Programming

Notes An additional feature of C++ abstract class is that it allows pure virtual methods in an abstract
class to contain an implementation in their declaring class. This mechanism providing fallback
or default behavior that a derived class can fall upon if needed.
The following code snippet defines an abstract class:

class Abst

{
public:
virtual void AbstFunc() = 0;
};
Function Abst::AbstFunc is a pure virtual function. Note that a function declaration cannot have
both a pure specifier and a definition. Due to this reason the following code is incorrect. The
compiler will issue an error message for this.
Class Abst
{

virtual void AbstFunc() { } = 0; //Incorrect! {} and =0 cannot be
used together
};
There are other restrictions also. You cannot use an abstract class as a parameter type, a function
return type, or the type of an explicit conversion, nor can you declare an object of an abstract
class. You can, however, declare pointers and references to an abstract class.
Care should be taken when cyclic inheritance is employed as shown in Figure 10.1.

Figure 10.1: Ambiguous Inheritance

In this case a compiler error takes place. The member function Func() of class D attempts to
access member data x of class A. The error results because the derived classes B and C derived
from base class A create copies of A called. Each of the copies have A member data and member
functions and each has one copy of member data x. When the member function of the class D
tries to access member data x, confusion arises as to which of the two copies it must access since
it has been derived from both derived classes.

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