Forward Declaration (Forward Declaration)

C ++ primer notes

You can declare a class without defining it:

Class Screen; // Declaration of the screen class

This statement is sometimes called a Forward Declaration (Forward Declaration ).ProgramThe class type screen is introduced. After the Declaration and definition, the class screen is an incomplete type, that is, it is known that the screen is a type, but it does not know which members are included.

Incomplete types can only be used in a limited way, and objects of this type cannot be defined. Incomplete types can only be used to define pointers and references pointing to this type, or to declare (rather than define) using this type as the parameter type or return functions of this type.