File inclusion errors (C++): undefined reference to ______

Question

Whenever I compile something that #includes a user-defined class, I get these compilation errors that always look like: main.cpp: undefined reference to Complex::Complex(double, double)

I've reduced the problem to a set of three extremely bare files: main.cpp, and for example, Complex.h and Complex.cpp. I still get undefined reference errors. I'm developing in Code::Blocks on Windows but I get the same thing using g++ in Ubuntu. Why does this happen? I've tried building Complex.cpp before main.cpp in Code::Blocks, and I've tried g++ main.cpp Complex.cpp as much as I've tried just g++ main.cpp. Same errors every time.

/*======== main.cpp ========*/
#include "Complex.h"

int main()
{
    Complex A(1.0, 1.0);
    return 0;
}

/*======== Complex.h ========*/
#ifndef _COMPLEX_H
#define _COMPLEX_H

class Complex
{
    public:
    double x, y;
    Complex(double real, double imag);
};

#endif

/*======== Complex.cpp ========*/
#include "Complex.h"

Complex::Complex(double real, double imag)
{
    x = real;
    y = imag;
}

ed: now I get different errors so I must be doing something completely wrong. Using the same code as above, I get:

main.cpp: in function 'int main()':
main.cpp:5:5: error: 'Complex' was not declared in this scope
main.cpp:5:13: error: expected ';' before 'A'

This is bizarre. Everything worked earlier when I had the class in a .cpp file, but that's "bad practice" so I moved my class definitions into .h files and kept the implementation in .cpp files, and now nothing works.

Answer 1

That's not a compilation error, it's a link error. You need to make sure to link all of your objects together. You can do that in a couple ways:

g++ main.cpp Complex.cpp

Should work fine (and does here when I tried with your example). You can also do it in steps:

g++ -c main.cpp
g++ -c Complex.cpp
g++ main.o Complex.o