Conditional compilation bug or feature ?

Hi,

I have a software that has 2 versions of an implemented algorithm, so it used different versions of the same class (in my case its SSE vs AVX, but really, it's not relevant) in 2 different compiler units, and suprisingly the linker take the initiative of taking an arbitrary one.

Is it a bug (that I reproduced as well in Microsoft and GCC compiler) or a feature (if that so can someone help me finding where this behavior is specified) ?

Here is how to reproduce the problem:

toto.h

#include <stdio.h>

#ifdef FLAG
class foo
{
public:
    void bar()
    {
        printf("FLAG\n");
    }
};
#else
class foo
{
public:
    void bar()
    {
        printf("NO FLAG\n");
    }
};
#endif

#ifdef FLAG
void call_foobar_FLAG()
#else
void call_foobar_NOFLAG()
#endif
{
    foo().bar();
}

toto_flag.cpp

#define FLAG
#include "toto.h"

toto_noflag.cpp

#include "toto.h"

main.cpp

void call_foobar_FLAG();
void call_foobar_NOFLAG();

int main(int argc, char ** argv)
{
    call_foobar_FLAG();
    call_foobar_NOFLAG();
}

We should expect as output

FLAG
NO FLAG

But we get

FLAG
FLAG

or

NO FLAG
NO FLAG

depending in which order object are linked...