Forward declaration outside class works, but not when nested

Question

Suppose I have two classes A and B employing the pimpl idiom. A provides the public API, holding a pointer to B. I get a compilation error when forward-declaring B within A, but not when declaring it outside.

Why would the latter not work? In both cases I am including b.hpp within a.cpp before calling any methods of B.

Forward declaration outside class A

This example works normally.

File a.hpp:

#ifndef _A_
#define _A_

#include <memory>

class B; // forward declaration, defined in a.cpp
class A {
public:
        A();
        ~A();
        void Hi();
private:
        std::unique_ptr< B > b_;
};

#endif

File a.cpp:

#include "b.hpp"
#include "a.hpp"

A::A() : b_( std::make_unique< B >() ) { }
A::~A() { }

void
A::Hi() {
        this->b_->Hi();
}

File b.hpp:

#ifndef _B_
#define _B_

class B {
public:
        void Hi();
};

#endif

File b.cpp:

#include "b.hpp"

#include <iostream>

void
B::Hi() {
        std::cout << "Hello World!" << std::endl;
}

Driver file hello.cpp:

#include "a.hpp"

int main() {
        A a;
        a.Hi();
        return 0;
}

Compiling: g++ hello.cpp a.cpp b.cpp -std=c++14

Forward declaration inside class A

Here I am moving the forward declaration of B within A.

File a.hpp:

#ifndef _A_
#define _A_

#include <memory>

class A {
public:
        A();
        ~A();
        void Hi();
private:
        class B; // forward declaration, defined in a.cpp
        std::unique_ptr< B > b_;
};

#endif

I receive the following compilation errors:

a.cpp: In member function ‘void A::Hi()’:
a.cpp:9:10: error: invalid use of incomplete type ‘class A::B’
  this->b_->Hi();
          ^
In file included from a.cpp:2:0:
a.hpp:12:8: error: forward declaration of ‘class A::B’
  class B;
        ^
In file included from /usr/include/c++/4.9/memory:81:0,
                 from a.hpp:4,
                 from a.cpp:2:
/usr/include/c++/4.9/bits/unique_ptr.h: In instantiation of ‘typename std::_MakeUniq<_Tp>::__single_object std::make_unique(_Args&& ...) [with _Tp = A::B; _Args = {}; typename std::_MakeUniq<_Tp>::__single_object = std::unique_ptr<A::B>]’:
a.cpp:4:36:   required from here
/usr/include/c++/4.9/bits/unique_ptr.h:765:69: error: invalid use of incomplete type ‘class A::B’
     { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
                                                                     ^
In file included from a.cpp:2:0:
a.hpp:12:8: error: forward declaration of ‘class A::B’
  class B;
        ^
In file included from /usr/include/c++/4.9/memory:81:0,
                 from a.hpp:4,
                 from a.cpp:2:
/usr/include/c++/4.9/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = A::B]’:
/usr/include/c++/4.9/bits/unique_ptr.h:236:16:   required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = A::B; _Dp = std::default_delete<A::B>]’
a.cpp:4:36:   required from here
/usr/include/c++/4.9/bits/unique_ptr.h:74:22: error: invalid application of ‘sizeof’ to incomplete type ‘A::B’
  static_assert(sizeof(_Tp)>0,

Answer 1

It is because you are not declaring the same class B. B in your second example is declared as a nested class. It has the scope A:: (as in it is actually named class A::B). But then you try to use it as ::B (as in global scope).

This is quite apparant from the error message:

error: invalid use of incomplete type ‘class A::B’

This cannot work. Forward declaring class B external to A is the correct way to implement this.