Polymorphism c++

In some books there is written that class that declares or inherits a virtual function is called a polymorphic class.

Class B doesn't have any virtual functions but passe开发者_StackOverflow社区s more than one is-a test.

Class C has one virtual function but doesn't inherit.

class A {};
class B : public A {};

class C
{
public:
    virtual void f () {}
};

is class B or C polymorphic ?

2003: 10.3/1 states, clearly:

A class that declares or inherits a virtual function is called a polymorphic class.

You actually said this yourself, word-for-word, so I don't really understand what the question is.

C (and its descendants, if you add any) is polymorphic; A and B are not.

^{Note that, in a wider OOP sense, you can always perform some "polymorphism" in that C++ always allows you to upcast; thus all objects that inherit can be treated as a different (but related) type.}

^{However, the term "polymorphic" is defined slightly differently in C++, where it has more to do with whether you can downcast as well. If you don't want to be confusing like the C++ standard, you might call this "dynamic polymorphism".}

Per the standard, "A class that declares or inherits a virtual function is called a polymorphic class."

Because neither A nor B declare or inherit a virtual function, they are not polymorphic. C declares a virtual function, so it is polymorphic.

class C is polymorphic, meaning that using dynamic_cast or typeid on a C& will do a runtime type check, and calling member functions through a C& or C* will use virtual dispatch.

(Of course, the as-if rule allows the compiler to avoid the runtime dispatch under some condition when it knows the runtime type in advance, such as when you just created the object.)

As @Bill mentioned in a comment, that isn't just what some books say, it's the definition of polymorphic class, found in the C++ standard (section 10.3, [class.virtual]):

Virtual functions support dynamic binding and object-oriented programming. A class that declares or inherits a virtual function is called a polymorphic class.