Boost C++ Libraries: Ticket #7300: Custom terminals are always nullary

attachment set

Andrey Semashev — Tue, 28 Aug 2012 18:07:41 GMT

attachment → terminal_test.cpp

Code sample that illustrates the problem

status changed; resolution set

Thomas Heller — Wed, 29 Aug 2012 06:04:03 GMT

status new → closed
resolution → fixed

Sorry for the delay ... looks like there is something wrong with my mail client ... I replied to your message to the mailing list already ...

The solution is here:

namespace my { template <typename> class output_terminal; }
namespace boost { namespace phoenix { namespace result_of {
    template< typename T >
    struct is_nullary< custom_terminal< my::output_terminal<T> > > :
        public mpl::false_
    {};

}}}

Put this at the beginning of your file and it compiles and runs. The is_nullary specialization needs to be wrapped inside another custom_terminal template in order to disambiguate from other rules.

Andrey Semashev — Wed, 29 Aug 2012 06:32:03 GMT

Thanks for the answer, it worked. Could the specialization in Boost.Phoenix be made more specific (for example, by using the nested void type technique) so that it is possible to create more generic specializations in user's code, like this:

template< typename T >
struct is_nullary< custom_terminal< T >, typename T::_is_my_terminal > :
    public mpl::false_
{
};