[ 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 ([class.mem]) 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 ([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.