Re: [Boost-users] [ptr_container] compiler error with BOOST_RESULT_OF_USE_DECLTYPE

Subject: Re: [Boost-users] [ptr_container] compiler error with BOOST_RESULT_OF_USE_DECLTYPE
From: Michel MORIN (mimomorin_at_[hidden])
Date: 2011-03-03 08:39:52

Thorsten Ottosen wrote:
> I was messing around with enable_if to see if we could detect the number of
> arguments. But I couldn't find a traits that does that yesterday ... now I
> see
>
> <boost/function_types/function_arity.hpp>
>
> could help. This way I think we can avoid instantiating result_of until it
> works.

Note that you cannot use SFINAE based on template `Fun`'s arity
since `Fun` is not a template parameter of void_ptr_indirect_fun's operator().

To avoid unnecessary instanciation of result_of,
we might need to make operator() a function template and
make its return type a dependent type of its template parameters.

The following code works fine:

#include <boost/static_assert.hpp>
#include <boost/type_traits/is_void.hpp>

/* ... */

template <typename Type, typename Dummy>
struct make_lazy
{
    typedef typename Type::type type;
};

template <typename Fun, typename Arg1, typename Arg2 = Arg1>
class void_ptr_indirect_fun
{
    /* ... */

    template <typename Dummy>
    typename make_lazy<
        boost::result_of<const Fun(const Arg1&)>
      , Dummy
    >::type
    operator()(const Dummy* r) const
    {
        BOOST_STATIC_ASSERT(boost::is_void<Dummy>::value);
        BOOST_ASSERT(r != 0);
        return fun(
            *static_cast<const Arg1*>(static_cast<const void*>(r))
        );
    }

    template <typename Dummy>
    typename make_lazy<
        boost::result_of<const Fun(const Arg1&, const Arg2&)>
      , Dummy
    >::type
    operator()(const Dummy* l, const Dummy* r) const
    {
        BOOST_STATIC_ASSERT(boost::is_void<Dummy>::value);
        BOOST_ASSERT(l != 0 && r != 0);
        return fun(
            *static_cast<const Arg1*>(static_cast<const void*>(l))
          , *static_cast<const Arg2*>(static_cast<const void*>(r))
        );
    }

    /* ... */
};

/* ... */

Regards,
Michel