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?

The error message doesn't match your code snippet.

Hans Passant
– Hans Passant

2011-01-05 04:11:10 +00:00
Commented Jan 5, 2011 at 4:11 — Hans Passant
– Hans Passant, Commented Jan 5, 2011 at 4:11

Logan Capaldo · Accepted Answer · 2011-01-05 01:57:27Z

18

Use the inline keyword.

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

answered Jan 5, 2011 at 1:57

Logan Capaldo

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12 Comments

Logan Capaldo Over a year ago

Not chapter and verse, but IIRC if you don't have inline it violates ODR (and if you do have inline but two or more different bodies it also violates ODR).

Sarfaraz Nawaz Over a year ago

inline is just a hint to the compiler, it may not inline sometimes.

Logan Capaldo Over a year ago

@Nawaz, yes for the purposes of optimization. But in this case we're leveraging it to specify linkage.

Lara Bailey Over a year ago

@Nawaz: No! inline is not just a hint to the compiler. This is a correct and appropriateuse of inline.

Lara Bailey Over a year ago

@Logan Capaldo: inline doesn't affect a function's linkage. inline functions still have external linkage by default.

|

wilhelmtell · Accepted Answer · 2011-01-05 02:00:05Z

7

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 << "]";
}

answered Jan 5, 2011 at 2:00

wilhelmtell

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Comments

Timo · Accepted Answer · 2011-01-05 02:36:10Z

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.

answered Jan 5, 2011 at 2:36

Timo

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Comments

Sarfaraz Nawaz · Accepted Answer · 2011-01-05 01:59:34Z

0

Define this function in .cpp file (not in .h file)

//yoursource.cpp
#include "yourheader.h"

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

answered Jan 5, 2011 at 1:59

Sarfaraz Nawaz

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