C++ in a Nutshell
| virtual specifier | Polymorphic function specifier or shared base class |
| function-specifier := virtual base-modifiers ::= virtual [ access-specifier ] access-specifier [virtual] |
The virtual keyword has two unrelated uses; it is used in virtual functions and virtual base classes:
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As a function specifier, virtual can be used only with a nonstatic member function. It makes the function and class polymorphic. A virtual function can be declared with = after the function header, which means the function is abstract. You cannot create an instance of a class with an abstract function; instead, a derived class must override the function. You can create an instance of the derived class.
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Using virtual as a base-class modifier means the base class subobject is shared when it is used more than once in an inheritance graph.
Example
struct shape { virtual void draw(canvas&) = 0; virtual void debug( ); }; struct square : virtual shape { virtual void draw(canvas&); virtual void debug( ); };
See Also
class , declaration , Chapter 6