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

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