5.2 KiB
[class.abstract]
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 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.
â end note]
A virtual function is specified as a pure virtual function by using apure-specifier ([class.mem]) in the function declaration in the class definition.
[Note 2:
Such a function might be inherited: see below.
â 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]).
â 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 virtualvirtual void draw() = 0; // pure virtual}; â end example]
[Note 4:
A function declaration cannot provide both a pure-specifier and a definition.
â end note]
[Example 2: struct C {virtual void f() = 0 { }; // error}; â 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.
â 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 functionab_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 ofcircle::draw() must be provided.
â 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.
â 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.