I am trying to understand the reason of the error I get while running main.cpp file. I have two class (foo and gee) included in main.cpp file.

here is the error I get:

foo.cpp: In member function ‘void foo::save(gee&)’:
foo.cpp:13:10: error: invalid use of incomplete type ‘class gee’
   13 |         i->addfoo(this);
      |          ^~
foo.h:7:7: note: forward declaration of ‘class gee’
    7 | class gee;
      |    

here is the declaration of foo class(foo.h):

#ifndef foo_H 
#define foo_H


#include <set>
#include <string>
class gee;
class foo
{
public:
    foo(std::string _str);
    void save(gee& geeobj);
private:
    std::string str;
    std::set<gee*> geelist;
};

#endif

here is the definition of foo class(foo.cpp):

#include "foo.h"

foo::foo(std::string _str)
{
    str = _str;
}

void foo::save(gee& geeobj)
{
    geelist.insert(&geeobj);
    for(auto i:geelist)
    {
        i->addfoo(this);
       // std::cout << "how to run i->addfoo(this); \n"; // this line can be run.
    }

    return;
}

here is the declaration of gee class(gee.h):

#ifndef gee_H
#define gee_H

#include <set>
#include <string>

class foo;
class gee
{
public:
    gee(std::string _name);
    void addfoo(foo& _foo_obj);

private:
    std::string name;
    std::set<foo*> foolist;
};

#endif

here is the definition of gee class(gee.cpp)

#include "gee.h"

gee::gee(std::string _name)
{
    name = _name;
}

void gee::addfoo(foo& _foo_obj)
{
    foolist.insert(&_foo_obj);
    return;
}

and here is the main file(main.cpp)

#include<iostream>
#include "gee.h"
#include "foo.h"

int main()
{
    gee gobj("geename");
    foo fobj("fooname");
    fobj.save(gobj); 
    return 0;
}

I am also trying to understand the dependency of these files and why I can't execute main.cpp like is it the linker problem can't find the addfoo function body. I thank you if you can give me in addition of the solution, an explanation of why we can't use this.

I am using g++ as compiler and to execute the main.cpp I just type these two line.

g++ gee.h foo.h
g++ main.cpp gee.cpp foo.cpp -o a