why defining a method with a return type as its class won't get me an 'incomplete type' error

Question

As far as I know when defining a function with an object return type, while the class is only in its forward declaration state like this:

class A; 

// forward declaration, which set A as an incomplete type

A foo(){...} 

//error: A is an incomplete type I know it works fine when it has a return type of a pointer or reference to that object.

But when I define a method with a return type as its class:

class B{
  public:
    B foo(){...}
}

It works perfectly fine.

I think when defining a method within the definition of class, the class is still an incomplete type. So I think it will prompt error similar to the former, but it didn't. Does anyone know why?

I have searched for quite some time before asking for help here. (I'm not good at English, so my description may get you confused. Sorry for that.)

Answer 1

When defining methods inside a class the class is treated as if it were a complete type, or else we would not be able to define inline methods. Also, if your class A were an incomplete type then the curiously recurring template pattern would not work either. Consider the following code:

template <typename T> struct base {};

struct derived : base<derived> {};  // We can use derived here
                                    // without any "incomplete type"
                                    // errors.

In other words: it's just the way the language works.

EDIT: See Mike Seymour's below for the relevant section of the C++ standard that mentions this behaviour.