C++ forward declaration error

Problem:

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()
{

}

Error:

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’

Solution:

Define the constructor for A AFTER the definition of struct B.