From: Guillaume Melquiond (guillaume.melquiond_at_[hidden])
Date: 2007-06-09 14:28:01
Le samedi 09 juin 2007 à 14:03 -0400, Gennadiy Rozental a écrit :
> Can anyone explain why unnamed namespace is a problem?
Inline functions referring to the content of an unnamed namespace will
break the one-definition rule, when their header is included several
times. Undefined behavior ensues.
The following example should make it clear:
// h.hpp
namespace { int a; }
inline int f() { return ++a; }
// a.cpp
#include "h.hpp"
void g() { f(); }
// b.cpp
#include "h.hpp"
void g();
int main() {
g();
return f();
}
Which value does this program return? Depending on the compiler and on
the level of optimization, it could be either 1 or 2.
If you use a named namespace instead, the behavior becomes deterministic
and the returned value is necessarily 2.
Best regards,
Guillaume