Re: [Boost-users] [Proto] Nested Types in Transforms

Subject: Re: [Boost-users] [Proto] Nested Types in Transforms
From: Eric Niebler (eric_at_[hidden])
Date: 2011-02-15 19:52:57

On 2/16/2011 7:04 AM, Hossein Haeri wrote:
> Dear all,
> 
> In my namespace arity_caller, I have a struct CanBeCalled<> such that CanBeCalled<T, mpl::int_<n> >::type is an mpl::bool_<> representing whether T can be called with n doubles. I also have the following grammar (in a file called 'EmtnGram.hpp') in which I am trying to employ this CanBeCalled<>::type:
> 
> struct EmtnShiftFObjGram:
>   boost::proto::and_
>   <
>     boost::proto::shift_right
>     <
>       EmtnTermOrGram,
>       boost::proto::terminal<boost::proto::_>
>     >,
>     boost::proto::if_
>     <
>       arity_caller::CanBeCalled
>       <
>         boost::proto::_value(boost::proto::_right),
>         EmtnTermOrGram(boost::proto::_left)
>       >::type
>     >
>   >
> {};

When you access a member of a class template, it causes the template to
be instantiated. CanBeCalled cannot legally be instantiated with two
function types. Hence the error.

Also, proto::if_ takes as it's template parameter a Transform. It should
be a transform that evaluates to a compile-time Boolean.

You can easily solve both problems by making the parameter to if_ an
ObjectTransform, as follows:

struct EmtnShiftFObjGram:
  boost::proto::and_
  <
    boost::proto::shift_right
    <
      EmtnTermOrGram,
      boost::proto::terminal<boost::proto::_>
    >,
    boost::proto::if_
    <
      arity_caller::CanBeCalled
      <
        boost::proto::_value(boost::proto::_right),
        EmtnTermOrGram(boost::proto::_left)
      >()
    >
  >
{};

Hope that helps,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com