Undefined reference using namespace C++ - c++

Question

When I want to run the code the compiler says undefined reference to math::calc
I read questions and answers about this problem at StackOverflow and it do not help me solve my problem.
Comp.h
#include <utility>
namespace math {
typedef std::pair<double,double> Comp;
double getFirst(Comp a);
...
}
Comp.cpp
#include "comp.h"
namespace math {
double getFirst(Comp a) {
return a.first;
}
...
}
Comp file: every function return Comp or double. I call function from cal.cpp file several times
cal.h
#include "comp.h"
#include "helper.h"
namespace math {
Comp calc(const string& str);
...
}
cal.cpp
#include "eval.h"
namespace math {
Comp calc(const string& str) {
...
}
}
Cal file: Some functions return with comp type, not just the cal function.
helper.h
namespace math {
...
}
helper.cpp
#include "helper.h"
namespace math {
...
}
helper file just contains few function that I calling from cal.cpp file. Each function is calling several times.
main.cpp
#include "calc.h"
int main() {
string str = " ";
pair<double,double> res = math::calc(str);
cout << res.first << " " << res.second << endl;
return 0;
}
In the whole project, I do not use any classes.
I included every file that I calling except the c++ original file.
I use the std namespace in each file but I do not write it here.
I absolutely no idea what could be the problem with my code.
If I also include cal.cpp in the main.cpp the code editor says undefined reference to each file that I calling from helper.h. I do not want to include cal.cpp in the main file I just mentioned it

Answer 1

You have to compile all your project's *.cpp files and then link them (results of compilation) properly - the linking process depends on the environment/IDE you're using, just look it up. If you're not linking it properly the final executable wont have all the functions definitions that requires, hence getting undefined reference.