[class.abstract] (original) (raw)
11 Classes [class]
11.7 Derived classes [class.derived]
11.7.3 Abstract classes [class.abstract]
[ Note
:
The abstract class mechanism supports the notion of a general concept, such as a shape, of which only more concrete variants, such ascircle 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 apure-specifier in the function declaration in the class definition.
[ Note
:
Such a function might be inherited: see below.
— end note
]
A class is an abstract classif it has at least one pure virtual function.
[ Note
:
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
]
[ Example
:
class point { };
class shape {
point center;
public:
point where() { return center; }
void move(point p) { center=p; draw(); }
virtual void rotate(int) = 0;
virtual void draw() = 0;
};
— end example
]
[ Note
:
A function declaration cannot provide both a pure-specifierand a definition.
— end note
]
[ Example
:
struct C {
virtual void f() = 0 { };
};
— end example
]
[ Note
:
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].
However, pointers and references to abstract class types can appear in such contexts.
— end note
]
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) { }
};
Since shape::draw() is a pure virtual functionab_circle::draw() is a pure virtual by default.
The alternative declaration,
class circle : public shape {
int radius;
public:
void rotate(int) { }
void draw();
};
would make class circle non-abstract and a definition ofcircle::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 ([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.