Re: [Boost-users] Using MPL for conditional compilation

From: David Abrahams (dave_at_[hidden])
Date: 2005-08-29 07:32:16

Ben Hutchings <ben.hutchings_at_[hidden]> writes:

> Alan M. Carroll wrote:
>
>>          typedef typename boost::mpl::if_c<IS_OUTER,
>>                   typename PAYLOAD::key_type,
>>                  PAYLOAD
>>         >::type testing;
>
> That can't work. All the template arguments to boost::mpl::if_c must be
> syntactically valid after substiution of C's template parameters, and
> clearly that won't be the case. I think you need to write your own
> selection template:
>
> template<typename T, bool> struct type_or_key_type
> { typedef T type; };
> template<typename T> struct type_or_key_type<T, true>
> { typedef typename T::key_type type; };
>
> then use
>
> typedef typename type_or_key_type<PAYLOAD, IS_OUTER>::type testing;

I'd normally solve it this way:

    template <class T> struct key_type
    {
        typedef typename T::key_type type;
    };

    typedef mpl::eval_if_c<
        IS_OUTER
      , key_type<PAYLOAD>
      , mpl::identity<PAYLOAD>
    >::type testing;

HTH,
-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com