C++ Nested classes forward declaration error

Question

I am trying to declare and use a class B inside of a class A and define B outside A.
I know for a fact that this is possible because Bjarne Stroustrup
uses this in his book "The C++ programming language"
(page 293,for example the String and Srep classes).

So this is my minimal piece of code that causes problems

class A{
struct B; // forward declaration
B* c;
A() { c->i; }
};

struct A::B { 
/* 
 * we define struct B like this becuase it
 * was first declared in the namespace A
 */
int i;
};

int main() {
}

This code gives the following compilation errors in g++ :

tst.cpp: In constructor ‘A::A()’:
tst.cpp:5: error: invalid use of undefined type ‘struct A::B’
tst.cpp:3: error: forward declaration of ‘struct A::B’

I tried to look at the C++ Faq and the closeset I got was here and here but
those don't apply to my situation.
I also read this from here but it's not solving my problem.

Both gcc and MSVC 2005 give compiler errors on this

Answer 1

The expression c->i dereferences the pointer to struct A::B so a full definition must be visible at this point in the program.

The simplest fix is to make the constructor of A non-inline and provide a body for it after the defintion of struct A::B.