11 Classes [class]

11.7 Derived classes [class.derived]

11.7.4 Abstract classes [class.abstract]

[Note 1:

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]

A virtual function is specified as a pure virtual function by using a pure-specifier ([class.mem]) in the function declaration in the class definition.

[Note 2:

Such a function might be inherited: see below.

— end note]

A class is an abstract class if it has at least one pure virtual function.

[Note 3:

An abstract class 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 ([basic.def], [class.mem]).

— end note]

A pure virtual function need be defined only if called with, or as if with ([class.dtor]), the qualified-id syntax ([expr.prim.id.qual]).

[Example 1: 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 4:

A function declaration cannot provide both a pure-specifier and a definition.

— end note]

[Example 2: struct C { virtual void f() = 0 { }; // error }; — end example]

[Note 5:

An abstract class type cannot be used as a parameter or return type of a function being defined ([dcl.fct]) or called ([expr.call]), except as specified in [dcl.type.simple].

Further, an abstract class type cannot be used as the type of an explicit type conversion ([expr.static.cast], [expr.reinterpret.cast], [expr.const.cast]), because the resulting prvalue would be of abstract class type ([basic.lval]).

However, pointers and references to abstract class types can appear in such contexts.

— end note]

A class is abstract if it has at least one pure virtual function for which the final overrider is pure virtual.

[Example 3: 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 6:

An abstract class can be derived from a class that is not abstract, and a pure virtual function can 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 ([class.virtual]) to a pure virtual function directly or indirectly for the object being created (or destroyed) from such a constructor (or destructor) is undefined.