Re: [boost] Mixing protocol

Subject: Re: [boost] Mixing protocol
From: David Abrahams (dave_at_[hidden])
Date: 2009-01-16 21:40:50

on Thu Jan 15 2009, Joel Falcou <joel.falcou-AT-u-psud.fr> wrote:

> Steven Watanabe a écrit :
>> What do you mean by mixed?
>>
>> result_of looks for a nested result or result_type only.  It is
>> unaffected by apply/type.
>>
>> MPL uses both apply and type and also has a metafunction called apply:
>>
> What I anted is having polymorph object function that also acts as a apply-based MPL
> metafunction
> to avoid code duplication for the meta-programmed parts.

You could use

    template <class F>
    struct result_of_adapter
    {
         template <class A0 = mpl::void_, class A1 = mpl::void_, class A2 = mpl::void_>
         struct apply
           : boost::result_of<F(A0,A1,A2)> {};

         template <class A0, class A1>
         struct apply<A0,A1,mpl::void_>
           : boost::result_of<F(A0,A1)> {};

         template <class A0>
         struct apply<A0,mpl::void_,mpl::void_>
           : boost::result_of<F(A0)> {};

         template <>
         struct apply<mpl::void_,mpl::void_,mpl::void_>
           : boost::result_of<F()> {};
    }

When you want to use a polymorphic function type F with mpl::apply, you can
wrap it in result_of_adapter:

     mpl::apply<result_of_adapter<F>, argt0, argt1>::type

You might also want to look at Boost.Fusion and
http://spirit.sourceforge.net/dl_docs/phoenix-2/libs/spirit/phoenix/doc/html/phoenix/starter_kit.html#phoenix.starter_kit.lazy_functions
both of which have to manage the same issue.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com