C++: Addition of ". cpp" implementations in Visual Studio and GCC

Question

In C++, what is observed in a quick internet search is the orientation that only ". h" files should be included. A sample of this can be observed here and especially here.

In Visual Studio, the inclusion of only ".h" with #include ... no problem, as long as the implementation files ". cpp" are included in the project. This ensures that they will be compiled and there will be a reference valid for class methods. However, this does not seem to be the case for GCC (g++). See the code below:

Mt. h

#ifndef MT_H
#define MT_H

namespace Subsistema {
  class Mt {
  public:
    int soma(int i, int d);
  };
}
#endif

Mt.cpp

#include "Mt.h"

namespace Subsistema {
    int Mt::soma(int i, int d) {
    return i + d;
  }
}

main.cpp

#include <iostream>
#include "Mt.h"

int main(int argc, char *argv[]){
  Subsistema::Mt * t = new Subsistema::Mt();

  std::cout << "Soma: " << t->soma(3, 5) << std::endl;

  delete t;

  return 0;
}

The above code will compile perfectly in Visual Studio even if there is no #include Mt.cpp, provided that it is included in the project. But it will NOT do so in g++, with the following error message:

/tmp/ccQVvF4r: In Function 'main':

main.cpp:7: Undefined Ference to 'Subsystem::Mt::soma(int, int)'

collect2: error: Ld returned 1 Exit status

This error could easily be solved by adding the macro #ifdef __GNUC__ , as follows:

#ifndef MT_H
#define MT_H

namespace Subsistema {
  class Mt {
  public:
    int soma(int i, int d);
  };
}

#ifdef __GNUC__
#include "Mt.cpp"
#endif

#endif

But the point is, this is a solution, but is the appropriate solution (best practice) to the situation? I need a solution, which as far as possible, independent of the compiler used.

NOTE: This question is closely related to How to include header and cpp without resulting in LNK2005 error in Visual Studio

Answer 1

The error message is not a build error. It is a linkage:

/tmp/ccQVvF4r.o: In function 'main':
main.cpp:7: undefined reference to 'Subsistema::Mt::soma(int, int)'
collect2: error: ld returned 1 exit status

It means that the linker the object file containing the method implementation is not found Subsistema::Mt::soma(int, int).

The Mt.cpp should be passed as a parameter to the compiler and not included through the directive #include.

This should compile your program without errors:

$ g++ main.cpp Mt.cpp -o teste