如何在不与好友友好或不声明指针的情况下在类主体中向前声明未嵌套的类

Question

I know two ways of declaring unnested class inside a class body.

First is:

class A
{
    friend class B; // B is forward-declared + friend
};

And the second is:

class A
{
    class B* ptr; // B is forward-declared + pointer
};

It's possible to forward-declare a class outside the class body, but for certain reason let's forget about this option;

So, I would like to make possible to declare such member:

class A
{
    /* somehow declare B in the body*/
    B* getInstanceB();
};

It's worth to mention, that: With declaring a pointer:

class A
{
    class B* pointer;
    B* getInstanceB(); //fine
};

With friend declaration:

class A
{
    friend class B;
    B* getInstanceB(); //GCC 5.3.0 issues error, MSVC compiles
};

I deeply regret that there is no such forward-declaring syntax like:

class ::[optional-namespace::]ClassName ;

which would distinguish nested class forward-declaration from unnested class forward-declaration. Is it possible to do that in current standard in some other way?

Answer 1

To forward-declare a class named B inside the class definition of A without having that class be nested within A , you can do something like the following:

class A {
    typedef class B B;
    // do something with B
};

An explanation can be found here .

A demonstration that this actually does work: http://coliru.stacked-crooked.com/a/27913ce45a572d36

Answer 2

So what you are trying to do is have an instance of a class within another class but have the definition after the "container class"?

//forward declaration of B
class B;

class A
{
  public:
    B* getInstanceB();

  private:
     B b_;

};

B* A::getInstanceB()
{
   return &b_;
}

//implementation of B
class B{};

like this?

HTH

Answer 3

I deeply regret that there is no such forward-declaring syntax like:

 class ::[optional-namespace::]ClassName; 

which would distinguish nested class forward-declaration from unnested class forward-declaration. Is it possible to do that in current standard in some other way?

No, I don't think there is.