Linker errors thrown when defining a function inside of a namespace?

Question

Why am I unable to define a function inside a namespace in a .cpp file (not main.cpp)?

For example, say I want to split my code up into 2 different files, main.cpp, and then lib.cpp (with an associated lib.h file).

The following setup throws a linker error: lib.h:

#ifndef LIB_H_
#define LIB_H_
namespace a{
void foo();
}
#endif

lib.cpp:

#include "lib.h"
using namespace a;
void foo(){
std::cout<<"Hello World!"<<std::endl;
}

main.cpp:

#include "lib.h"
int main(){
a::foo();
return 0;
}

Almost any other variation I can think of works however.

• Defining the function in the lib.h file instead of lib.cpp works
• Removing the namespace works
• Keeping the namespace and instead having foo as a public method in a class which I instantiate in the main function, and then call, works

It seems to me like this might be a convention thing? To encourage people to either use classes or to define the function fully in a .h file? The linker error I'm getting is "undefined symbols(a::foo()) for architecture..."

Why does this happen? I'm on a mac using CLion.

Answer 1

using namespace a; is not a way to define a function inside a namespace.

The syntax is the same as with the declaration: namespace a { ... }

#include "lib.h"
namespace a {
    void foo(){
        std::cout<<"Hello World!"<<std::endl;
    }
}

For more details refer to: Namespaces.