JOAQUIN LOPEZ MU?Z wrote:
Hello,
In a post from yesterday (see http://tinyurl.com/hzk5w), Alexei Alexandrov found a rather serious problem with is_convertible in at least MSVC 7.1 and Intel 9.0, namely that is_convertible<From,To> fails to compile when
From is non-copyable, emitting an error message like this:
boost/boost/type_traits/is_convertible.hpp(254): error: class "XXX" has no suitable copy constructor BOOST_STATIC_CONSTANT(bool, value = ...
where XXX is the From argument. Needless to say, some fix for this issue would be most welcome. I'm sorry I cannot provide any more help than a mere report of the problem.
Thank you,
Using the trivial code below I'm *unable* to reproduce this: it's compiles cleanly with all the compilers I've tried (including Intel 9 and VC7.1). However.... there is a long history of is_convertible doing strange things under very specific circumstances with VC7.1 (and of course Intel emulate VC's bugs). Indeed the VC-specific implementation relies on undefined behaviour: passing UDT's through a function with elipsis, and it's this that appears to be causing the problem in this case. There are alternative implementations of is_convertible - almost one per compiler in fact, which shows how much trouble it's caused in the past - that do not rely on this undefined behaviour, and do not pass UDT's through elipsis, and so should not have this problem. Unfortunately these implementations could not be used with VC++ because even though they pass all our type-traits regression tests, they cause strange and unexplained failures deep within certain instantiation contexts. Sound familiar? Anyway, trivial test case follows, John. #include <boost/type_traits.hpp> #include <boost/noncopyable.hpp> class bar : private boost::noncopyable { }; template <bool b> class foo { public: static const bool value = b; }; typedef foo< ::boost::is_convertible<bar, int>::value> foo_t; const bool b = foo_t::value;