C++, good old LNK1169 (and LNK2005) errors

Question

I have 4 files, 2 headers and 2 cpp files.

Header file one:

#ifndef complex_2
#define complex_2
#include
#include

using         


        

Answer 1

When you define a standalone function (with a body) in header file you must use inline (functions defined within class definition are inline by default).

E.g. in header file one must be :

inline ostream& operator<< (ostream &mm, const complex &c){
    if(c.get_im() >= 0){
        mm << "(" << c.get_re() << " + " << c.get_im() << "i)" << endl;
    }
    if(c.get_im() < 0){
        mm << "(" << c.get_re() << " - " << -(c.get_im()) << "i)" << endl;
    }
    return mm;
}

Otherwise you will define this function in each translation unit that includes that header and thus may get multiple definition error.