Global function definition in header file - how to avoid duplicated symbol linkage error

Question

I have the following code in a header only file.

#pragma once

class error_code {
public:
    unsigned __int64 hi;
    unsigned __int64 lo;    
};

std::ostream& operator<< (std::ostream& o, const error_code& e) {
    return o << "[" << e.hi << "," << e.lo << "]";
}

I get linkage error, when there are 2 cpp in the project include this header file.

error LNK2005: "class error_code __cdecl operator|(class error_code const &,class ViTrox::error_code const &)" (??U@@YA?AVerror_code@0@ABV10@0@Z) already defined in xxx.obj

I know I can resolve this problem, if I move the definition of operator<< to a cpp file, or to a DLL file.

However, I just would like to have them in a SINGLE header file. Is there any technique to achieve so? Or must I separate the definition to another file?

Answer 1

Use the inline keyword.

inline std::ostream& operator<< (std::ostream& o, const error_code& e) {
    return o << "[" << e.hi << "," << e.lo << "]";
}

Answer 2

Either make the function inline:

inline std::ostream& operator<< (std::ostream& o, const error_code& e) {
    return o << "[" << e.hi << "," << e.lo << "]";
}

or make it a template function:

template<class Ch, class Tr>
std::basic_ostream<Ch,Tr>& operator<< (std::basic_ostream<Ch,Tr>& o,
                                       const error_code& e) {
    return o << "[" << e.hi << "," << e.lo << "]";
}

Answer 3

You can make the function static. It specifies internal linkage, so the linker won't care if the function is already defined in other translation units.

Or, as already mentioned, you can make it inline. It still has external linkage, but the standard allows external inline functions to have a definition in multiple translation units.