Re: [boost] [mpl] has_template_xxx patch

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2007-03-20 10:02:35

On 3/20/07, Daniel Walker <daniel.j.walker_at_[hidden]> wrote:
> Hello,
>
> I would like a Metafunction similar to the one created by
> BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF but that would detect nested
> template members rather than nested typedefs and classes. It would
> allow you to do something like the following:
>
> #include <utility>
> #include <boost/mpl/assert.hpp>
> #include <boost/mpl/has_template_xxx.hpp>
> #include <boost/utility/result_of.hpp>
>
> // Create has_template_result to detect nested members like
> // template<class T> struct result {}.
> // The first macro argument is the member name and the second argument
> // is the number of template parameters.
> BOOST_MPL_HAS_TEMPLATE_XXX_TRAIT_DEF(result, 1)
>
> struct pair_maker {
>     template<typename F> struct result;
>     template<typename F, typename Arg0, typename Arg1>
>     struct result<F(Arg0, Arg1)> {
>         typedef std::pair<Arg0, Arg1> type;
>     };
>
>     template<typename Arg0, typename Arg1>
>     typename result<pair_maker(Arg0, Arg1)>::type
>     operator()(Arg0 a0, Arg1 a1) const
>     {
>         return std::make_pair(a0, a1);
>     }
> };
>
> int main()
> {
>     using namespace boost;
>
>     typedef char value_type;
>     BOOST_MPL_ASSERT((
>         has_template_result<
>             pair_maker,
>             pair_maker(value_type, value_type)
>         >
>     ));
>
>     typedef result_of<
>         pair_maker(value_type, value_type)
>     >::type result_type;
>
>     value_type x, y;
>     pair_maker f;
>     result_type result = f(x, y);
> }
>
> The attached patch implements this. Apply with 'patch -p0 <
> djw_has_template.patch' from the boost root directory. It's based on
> the implementation of BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF and works
> basically the same way. The patch creates two files
> boost/mpl/has_template_xxx.hpp and
> boost/mpl/aux_/config/has_template_xxx.hpp. Similarly to has_xxx, when
> the maco BOOST_MPL_CFG_NO_HAS_TEMPLATE_XXX is defined the
> has_template_xxx::value is always false or a user supplied default. At
> this time, I'm unable to write all the various compiler workarounds
> that boost/mpl/has_xxx.hpp includes. BOOST_MPL_CFG_NO_HAS_TEMPLATE_XXX
> is defined for every workaround mentioned in boost/mpl/has_xxx.hpp.
> I've tested it with gcc 3.3, 3.4, 4.0, and 4.1 and it seems to work
> fine. If there's any interest, let me know and I'll supply
> documentation and tests.
>

One up for this. I've needed it to detect the 'sig<...>' metafunctions
to distinguish Boost.Lambda function objects from other polymorphic
function objects.

gpd