[boost] Concept based overloading and the ODR

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-07-10 13:31:42

AMDG

I'm looking at two recent posts:
http://comments.gmane.org/gmane.comp.lib.boost.devel/177370
http://comments.gmane.org/gmane.comp.lib.boost.user/37476

Either of these is fine by itself, but putting them both
together makes it very easy to cause silent ODR
problems.  For example in translation unit a.cpp we
#include  "boost/tuple/tuple_io.hpp"
and translation unit b.cpp we forget.  Then, along comes
boost::any which detects operator<< in translation
unit a.cpp, but uses its default in translation unit
b.cpp.  Ka-Boom.

This is not a huge problem at the moment, because it's
quite difficult to make the detection bullet-proof,
but I'm very concerned because C++0x concepts
are supposed to make such overloading easy.

In Christ,
Steven Watanabe