David Abrahams wrote:
"Eric Niebler" <eric@boost-consulting.com> writes:
David Abrahams wrote:
Eric Niebler <eric@boost-consulting.com> writes:
Put the overload in an associated namespace of the type. Or for maximum portability, you could put the overload at global scope.
Really? If invoked from within a template, global scope overloads won't be found unless an argument is in the global namespace or the overloads happen to appear before the point of definition of the template.
That was the idea, yes.
If you want a way to make this portable, consider adding a dummy parameter that comes from namespace Boost and asking people to overload there (the Ramey trick).
I'm not sure how this helps on compilers that don't do ADL. This is a macro. BOOST_FOREACH(foo, bar) will expand to something that makes a non-qualified call to boost_foreach_has_cheap_copy *in the context of the BOOST_FOREACH invocation*. This could be any context, in any namespace.
Right. Including from within a template, which could foil lookup in the global namespace.
If I want this to work on compilers that don't do ADL, requiring people to make their overloads visible at global scope seems necessary, but I've likely overlooked something.
namespace boost { template <class T> struct foreach_has_cheap_copy { char (& test(...)) [2]; char test(mpl::true_*); typedef mpl::bool_< sizeof( test( boost_foreach_has_cheap_copy( (T*)0, boost::whatever()) )) > type; }; }
Now all compilers will look in namespace boost, whether or not they support ADL.
I thought of something like this shortly after I sent the email. Sadly, I cannot get VC6 to eat it. "error C2133: '$S38' : unknown size". Pbbth. I tried a few variations, and then gave up. Templates don't "foil lookup in the global namespace." They merely require that the overload has been seen already. I have no problem requiring users of broken compilers to put the overload where BOOST_FOREACH can find it. Users of non-broken compilers can put the overload in an associated namespace and be happy. -- Eric Niebler Boost Consulting www.boost-consulting.com