C ++ Polymorphism

Question

Some books say that a class that declares or inherits a virtual function is called a polymorphic class.

Class B does not have virtual functions, but more than one passes - this is a test.

Class C has one virtual function, but does not inherit.

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

is class B or C polymorphic?

Answer 1

2003: 10.3/1 clearly indicates:

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

You said it yourself, literally, so I really don’t understand what the question is.

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

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^{Note that in the broader sense of OOP, you can always perform some “polymorphism” in that C ++ always allows you to speed up; thus, all objects that inherit can be considered as different (but related) types.}

^{However, the term “polymorphic” is somewhat different in C ++, where it has more in common with whether it can also be flushed down. If you don't want to be misled like the C ++ standard, you can call this "dynamic polymorphism."}

Answer 2

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

Since neither A nor B declares or inherits a virtual function, they are not polymorphic. C declares a virtual function, so it is polymorphic.

Answer 3

class C is polymorphic, which means that using dynamic_cast or typeid in C& will perform runtime type checking, and member functions through C& or C* will use virtual send.

(Of course, the as-if rule allows the compiler to avoid scheduling during execution of some condition when it knows in advance the type of execution, for example, when you just created an object.)

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As mentioned in a comment by @Bill, this is not what some books say, it is a definition of a 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.