John C. Femiani wrote:
Bruno Lalande wrote:
Hi John,
Thanks to have taken the time to test my code. You had the same result as Barend, who was able to reproduce the problem with GCC cygwin (3.4.4) but not MSVC. But the strange thing is that for me, the problem occurs for any compiler (MSVC or GCC). I really don't understand. Here is the list of compilers tested: - Barend: MSVC 8 Express (succeeds), cygwin GCC 3.4.4 (fails) - Me: MSVC 8 (fails), MSVC 9 Express (fails), GCC 4.2.3 (fails)
I don't think there can be any functional difference between the express and normal version of the same MSVC so I don't understand. Could you precise the exact version you used?
I've attached my MSVC compilation results to the mail.
Bruno
I think boost is using the fact that the compiler should fail to define a nested enum when a concept assert fails, but that SFINAE should not consider that an error. It looks like it is doing this so that it can chain together many concept checks. I think this must be a bug, so I added a ticket http://svn.boost.org/trac/boost/ticket/2137.
OK, so apparently the ticket was invalid because BOOST_CONCEPT_REQUIRES does not actually use SFINAE, for technical reasons, so if we want concept based overloading it just wont work. I think that in order to approximate that you will need to create a metafunction template<class T> struct is_dummy: ::boost::mpl::false_ {}; template<template<class T> > struct is_dummy<foo<T> > : ::boost::mpl::true_ { BOOST_CONCEPT_ASSERT( (DummyConcept<T> )); }; template<typename T> typename ::boost::enable_if<is_dummy<T>, void>::type func(T) { std::cout << "first overload" << std::endl; } and make that part of the concept, so you can use it with enable_if. --John