Question

File ex1.hpp (definition of template class):

#ifndef EX1_HPP
#define EX1_HPP

#include "ex2.hpp"

template <class T>
class Ex1{
public:
    void Ex1method(){
        Ex2 a; // using nontemplate class object
        a.Ex2method();
    }
};

#endif

File ex2.hpp (definition of nontemplate class):

#ifndef EX2_HPP
#define EX2_HPP

#include "ex1.hpp"

class Ex2{
public:
    void Ex2method();
};

#endif

File ex2.cpp (definition of nontemplate class method):

#include "ex2.hpp"

void Ex2::Ex2method()
{
    Ex1<int> e; // using template class object
    e.Ex1method();
}

Compilation error:

ex1.hpp:10:9: error: ‘Ex2’ was not declared in this scope
ex1.hpp:10:13: error: expected ‘;’ before ‘a’
ex1.hpp:11:9: error: ‘a’ was not declared in this scope

How can be such circular dependency between nontemplate class and template class resolved? I cannot be definition of nontemplate class methods to implementation file, because it will cause linker error. If I place forward declaration of Ex2 class in file ex1.hpp, then error is:

error: invalid use of incomplete type ‘struct Ex2’
error: forward declaration of ‘struct Ex2’

Answer 1

Don’t include ex1.hpp in ex2.hpp, it is not necessary there. You don’t have a circular dependency with your headers.

Include both headers (or just ex1.hpp) in your .cpp file and all should be good.