13 Derived classes [class.derived]

[ Note: The abstract class mechanism supports the notion of a general concept, such as a shape, of which only more concrete variants, such as circle and square, can actually be used. An abstract class can also be used to define an interface for which derived classes provide a variety of implementations.  — end note ]

An abstract class is a class that can be used only as a base class of some other class; no objects of an abstract class can be created except as subobjects of a class derived from it. A class is abstract if it has at least one pure virtual function. [ Note: Such a function might be inherited: see below.  — end note ] A virtual function is specified pure by using a pure-specifier in the function declaration in the class definition. A pure virtual function need be defined only if called with, or as if with ([class.dtor]), the qualified-id syntax ([expr.prim]). [ Example:

class point { /* ... */ };
class shape {                   // abstract class
  point center;
public:
  point where() { return center; }
  void move(point p) { center=p; draw(); }
  virtual void rotate(int) = 0; // pure virtual
  virtual void draw() = 0;      // pure virtual
};

 — end example ] [ Note: A function declaration cannot provide both a pure-specifier and a definition  — end note ] [ Example:

struct C {
  virtual void f() = 0 { };     // ill-formed
};

 — end example ]

An abstract class shall not be used as a parameter type, as a function return type, or as the type of an explicit conversion. Pointers and references to an abstract class can be declared. [ Example:

shape x;                        // error: object of abstract class
shape* p;                       // OK
shape f();                      // error
void g(shape);                  // error
shape& h(shape&);               // OK

 — end example ]

A class is abstract if it contains or inherits at least one pure virtual function for which the final overrider is pure virtual. [ Example:

class ab_circle : public shape {
  int radius;
public:
  void rotate(int) { }
  // ab_­circle​::​draw() is a pure virtual
};

Since shape​::​draw() is a pure virtual function ab_­circle​::​draw() is a pure virtual by default. The alternative declaration,

class circle : public shape {
  int radius;
public:
  void rotate(int) { }
  void draw();                  // a definition is required somewhere
};

would make class circle non-abstract and a definition of circle​::​draw() must be provided.  — end example ]

[ Note: An abstract class can be derived from a class that is not abstract, and a pure virtual function may override a virtual function which is not pure.  — end note ]

Member functions can be called from a constructor (or destructor) of an abstract class; the effect of making a virtual call to a pure virtual function directly or indirectly for the object being created (or destroyed) from such a constructor (or destructor) is undefined.