派生类的组成

Question

I have a question regarding composition and inheritance in C++:

I have a base class 'A' and a derived class 'B' ( 'B' inherits members from 'A' ), is it possible use 'B' as a member of 'A' by composition?

Answer 1

As to "A has B member, B derived from A": Practically, you cannot declare such a thing. If class A has a member of type B, then B needs to be declared before A. If B uses A as a base, A must be declared before B.

As to composition, if you wanted to do such a thing, you could have to use indirection (pointer, smart-pointer, etc).

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

One might argue that (in C++) using pointers is not composition; however, in Java, all Objects are "pointers", so composition is:

class A {
  B b;
}
class B extends A {
}

Answer 2

However you may embed a B* field. Hence a const B& should be possible too, and behave like a B . (Mind I am now a sandbox java programmer.)

class B;
class A {
    B& b;
};

BTW you might not even embed an A in an A.