ceph/src/boost/libs/type_erasure/include/boost/type_erasure/deduced.hpp

   1 // Boost.TypeErasure library
   2 //
   3 // Copyright 2011 Steven Watanabe
   4 //
   5 // Distributed under the Boost Software License Version 1.0. (See
   6 // accompanying file LICENSE_1_0.txt or copy at
   7 // http://www.boost.org/LICENSE_1_0.txt)
   8 //
   9 // $Id$
  10
  11 #ifndef BOOST_TYPE_ERASURE_DEDUCED_HPP_INCLUDED
  12 #define BOOST_TYPE_ERASURE_DEDUCED_HPP_INCLUDED
  13
  14 #include <boost/mpl/eval_if.hpp>
  15 #include <boost/mpl/identity.hpp>
  16 #include <boost/mpl/set.hpp>
  17 #include <boost/mpl/empty.hpp>
  18 #include <boost/type_erasure/detail/get_placeholders.hpp>
  19 #include <boost/type_erasure/placeholder.hpp>
  20
  21 namespace boost {
  22 namespace type_erasure {
  23
  24 /**
  25  * A placeholder for an associated type.  The type corresponding
  26  * to this placeholder is deduced by substituting placeholders
  27  * in the arguments of the metafunction and then evaluating it.
  28  *
  29  * When using @ref deduced in a template context, if it is possible for
  30  * Metafunction to contain no placeholders at all, use the nested type,
  31  * to automatically evaluate it early as needed.
  32  */
  33 template<class Metafunction>
  34 struct deduced : ::boost::type_erasure::placeholder
  35 {
  36     typedef typename ::boost::mpl::eval_if<
  37         ::boost::mpl::empty<
  38             typename ::boost::type_erasure::detail::get_placeholders<
  39                 Metafunction,
  40                 ::boost::mpl::set0<>
  41             >::type
  42         >,
  43         Metafunction,
  44         ::boost::mpl::identity<
  45             ::boost::type_erasure::deduced<Metafunction>
  46         >
  47     >::type type;
  48 };
  49
  50 }
  51 }
  52
  53 #endif