[Boost-users] Problems using mpl::if_ and function_types::result_type

Subject: [Boost-users] Problems using mpl::if_ and function_types::result_type
From: Allan Nielsen (a_at_[hidden])
Date: 2011-11-24 08:25:28

Hi

I'm new to the boost::mpl library, and have some "beginners-problems"

template < typename F >
struct A {
    typedef boost::function_types::parameter_types<F> P;
    typedef typename boost::function_types::result_type<F>::type R;

    typedef typename boost::mpl::if_< boost::is_same< R, void >,
                                      boost::function< void ( void ) > ,
                                      boost::function< void ( R ) > >::type TTT;
    A() {
    }
};

int main(int argc, const char *argv[])
{
    A<int(int, float)>  ok; // working
    A<void(int, float)> compile_error; // non-working

    return 0;
}

xxx.cxx: In instantiation of ‘A<void(int, float)>’:
xxx.cxx:108:25:   instantiated from here
xxx.cxx:100:77: error: invalid parameter type
‘boost::mpl::aux::wrapped_type<boost::mpl::aux::type_wrapper<void>
>::type’
xxx.cxx:100:77: error: in declaration ‘A<F>::TTT’

What is the problem here, and how can I solve it.

To my understanding only the selected part of mpl::if_ should be
evaluated by the compiler....

Regards
Allan W. Nielsen