Class static function external linkage

Question

How can achieve external linkage for class static functions when the compiler thinks the code is local linkage or inline?

Consider three files:

traits.h:

template <typename T>
struct Traits
{
    static int InlineFunction(T);
    static int Function(T);
};

traitsimp.cpp:

#include "traits.h"
template <>
struct Traits<int>
{
    static int InlineFunction(int) { return 42; }
    static int Function(int);
};

int Traits<int>::Function(int i) { return i; }

main.cpp:

#include "traits.h"
int main()
{
    int result = Traits<int>::Function(5);
    result = Traits<int>::InlineFunction(result);
    return 0;
}

When compiled receives:

$ g++ traitsimp.cpp main.cpp -o traitstest
/tmp/cc6taAop.o: In function `main':
main.cpp:(.text+0x1b): undefined reference to `Traits<int>::InlineFunction(int)'
collect2: ld returned 1 exit status

How do I convince the compiler to give InlineFunction external linkage while still writing the function within the class definition?

Updated. I'd like to not be forced to write the functions outside of the class definition. And even more importantly, why does it matter? — Bill Door, Mar 13 '13 at 04:32
afaik, it does not matter if you write within the class or externally like the implementation file. — Aniket Inge, Mar 13 '13 at 04:34
See also: http://stackoverflow.com/questions/4446968/template-specialization-multiply-defined-symbols/4447405#4447405 — Keith, Mar 13 '13 at 04:35

Keith · Answer 1 · 2013-03-13T04:44:27.963

I think you need to decide whether to have external linkage and define out of line or keep it inline and effectively just have internal linkage. Is there a reason not to define your Traits<int> in a header? Note that the full specialization is no longer a template, so there is really no difference to how it is handled versus a non-templated function.

Edited: Tried this in VS2010, and it may still not suit, but you could put code in traitsimp.cpp that takes the address of the method:

auto getAddress = &Traits<int>::InlineFunction;

This forces the method to get an address. I'm a bit rusty on whether this behaviour would be standard or not.

Class static function external linkage

1 Answers1