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From: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2007-03-20 09:51:37


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.

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;
            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.

Daniel Walker

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