Should I include the header containing the definition of a namespace in source files?

Question

If I have a namespace defined in a header file and in some source files I don't include that header but instead define a namespace with the same name as the one in the header and declare the same members as in the header file then add definitions then am I defining a new namespace or opening the existing one?

//file.h
#ifndef FILE_H_INCLUDED
#define FILE_H_INCLUDED

namespace mylibNS{
    void foo();
    void bar();
}

#endif // FILE_H_INCLUDED

// foo.cpp
#include <iostream>
//#include "file.h"

namespace mylibNS{
    void foo();
}

void mylibNS::foo(){
    std::cout << "foo()\n";
}

// bar.cpp
#include <iostream>
// #include "file.h"

namespace mylibNS{
    void bar();
}

void mylibNS::bar(){
    std::cout << "bar()\n";
}

// main.cpp
#include <iostream>
using namespace std;
#include "file.h"

int main(int argc, char* argv[]){

    mylibNS::bar();
    mylibNS::foo();
}

• The code above works fine although in source files foo.cpp and bar.cpp I didn't include the header file.h containing the definition of namespace mylibNS ? So in those source files am I opening an existing namespace or defining a new one in each file? The program works just fine so should I do it this way or stick to the standard (opening the namespace and adding to it)?

• Should I include file.h in foo.cpp and bar.cpp and main.cpp ?

Answer 1

It is common practise to include file.h in any file where foo or bar are needed and drop the duplicate declarations. Then, if a function's signature changes (or if a new function is added) it only needs to be changed in one place. It also avoids the danger of declaring a function in two different places with two different signatures.

Answer 2

Should I include the header containing the definition of a namespace in source files?

Technically, including a header file is the same thing as copying the content of that file into the other and thus it is entirely possible to write a program without using any header files. I suggest that you don't do that.

Every declaration that you use in more than one translation unit, you should declare in a header file, and include in each translation unit that use the declaration.

So in those source files am I opening an existing namespace

Namespaces with same name in different translation units are the same namespace. Only an anonymous namespace is unique to the translation unit where it is declared.