Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards


// Boost.TypeErasure library
// Copyright 2011 Steven Watanabe
// Distributed under the Boost Software License Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// $Id: deduced.hpp 83393 2013-03-10 03:48:33Z steven_watanabe $


#include <boost/mpl/eval_if.hpp>
#include <boost/mpl/identity.hpp>
#include <boost/mpl/set.hpp>
#include <boost/mpl/empty.hpp>
#include <boost/type_erasure/detail/get_placeholders.hpp>
#include <boost/type_erasure/placeholder.hpp>

namespace boost {
namespace type_erasure {

 * A placeholder for an associated type.  The type corresponding
 * to this placeholder is deduced by substituting placeholders
 * in the arguments of the metafunction and then evaluating it.
 * When using @ref deduced in a template context, if it is possible for
 * Metafunction to contain no placeholders at all, use the nested type,
 * to automatically evaluate it early as needed.
template<class Metafunction>
struct deduced : ::boost::type_erasure::placeholder
    typedef typename ::boost::mpl::eval_if<
            typename ::boost::type_erasure::detail::get_placeholders<
    >::type type;