Hello John, Thursday, March 24, 2005, 8:23:00 PM, you wrote:
The code below does not compile, when, I believe, it should. Compiler says that you can't instantiate abstact class.
I'm really surprised - it is trivial use case, but I did not find a problem report. Do I miss something?
JM> This is a tricky one; first the headline news: current cvs will compile JM> that, and will report false whenever the "To" type is abstract, but only if JM> you have a newish compiler that supports is_abstract. The reason for JM> returning false, is that nothing is ever convertible to an abstract type, JM> since no object of an abstract type can ever be created. JM> Now for the problem: is general is_convertible<A,A> may not compile if A's JM> copy-constructor is non-public. We'd love to be able to fix that, but no JM> one knows how to detect whether a member is accessible or not. JM> HTH, JM> John. JM> _______________________________________________ JM> Unsubscribe & other changes: JM> http://lists.boost.org/mailman/listinfo.cgi/boost from is_convertable.hpp template< typename From > struct does_conversion_exist { template< typename To > struct result_ { static no_type BOOST_TT_DECL _m_check(...); static yes_type BOOST_TT_DECL _m_check(To); static From _m_from; enum { value = sizeof( _m_check(_m_from) ) == sizeof(yes_type) }; }; }; if static From _m_from; is replaced by static const From& make_from(); it'll work with abstract types template< typename From > struct does_conversion_exist { template< typename To > struct result_ { static no_type BOOST_TT_DECL _m_check(...); static yes_type BOOST_TT_DECL _m_check(To); static const From& make_from(); enum { value = sizeof( _m_check(make_from()) ) == sizeof(yes_type) }; }; }; -- Best regards, Nick mailto:kolach56@yandex.ru