How to declare/define interdependent templates in C++?

Question

Usually in C++ when I need interdependencies between classes, I use forward declarations in the header files and then include both header files in each cpp file.

How

Answer 1

You can utilize fine-grained headers:

//  A.forward.hpp
template struct A;

//  A.decl.hpp
#include "A.forward.hpp"
#include "B.forward.hpp"

template struct A
{
    void RunA(B *pB);
};

//  A.impl.hpp
#include "A.decl.hpp"
#include "B.hpp"

template void A< T >::
RunA(B *pB)
{
    // need to do something to B here
}

//  A.hpp // this one should be included by code using A
#include "A.decl.hpp"
#include "A.impl.hpp"

//  B.forward.hpp
template struct B;

//  B.decl.hpp
#include "B.forward.hpp"
#include "A.forward.hpp"

template struct B
{
    void RunB(A *pA);
};

//  B.impl.hpp
#include "B.decl.hpp"
#include "A.hpp"

template void B< T >::
RunB(A *pA)
{
    // need to do something to A here
}

//  B.hpp // this one should be included by code using B
#include "B.decl.hpp"
#include "B.impl.hpp"

Obviously all of these headers also need some sort of header guards that I omitted here. Important notice: .impl.hpp headers are considered internal and should never be used by the outside code while .forward.hpp, .decl.hpp and .hpp can be used anywhere.

This approach does introduce circular include dependencies, however this does not lead to problems: code structure produced by inclusion of headers ensures that code parts are included in the following order: forward declarations, class definitions, methods definitions.