Examples of Errors using Abstract Classes

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*

The following example shows an attempt to create an object of an abstract class type.

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class AB             // abstract class
{
public:
      virtual void f()= 0; // pure virtual member function
};
class D: public AB
{
public:
      void f();
};
// ...
void main ()
{
      D d;
      d.f() ;     // calls D::f()
      AB ab;      // error, cannot create an object of an
                  // abstract class type
}

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*

The following example shows an attempt to create an object of a class derived from an abstract class, but that does not redefine the pure virtual function of that abstract class.

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For example:

class AB // abstract class
{
public:
      virtual void f()= 0; // pure virtual member function
};
class D2: public AB
{
      int a,b,c;
public:
      void g();
};
// ...
void main ()
{
      D2 d;
      // error, cannot declare an object of abstract class D2
}

To avoid the error in the above example, provide a declaration of D2 ::f().