metaparse/v1/optional.hpp

0001 #ifndef BOOST_METAPARSE_V1_OPTIONAL_HPP
0002 #define BOOST_METAPARSE_V1_OPTIONAL_HPP
0003
0004 // Copyright Abel Sinkovics (abel@sinkovics.hu)  2015.
0005 // Distributed under the Boost Software License, Version 1.0.
0006 //    (See accompanying file LICENSE_1_0.txt or copy at
0007 //          http://www.boost.org/LICENSE_1_0.txt)
0008
0009 #include <boost/metaparse/v1/is_error.hpp>
0010 #include <boost/metaparse/v1/accept.hpp>
0011
0012 #include <boost/mpl/if.hpp>
0013
0014 namespace boost
0015 {
0016   namespace metaparse
0017   {
0018     namespace v1
0019     {
0020       template <class P, class Default = void>
0021       struct optional
0022       {
0023         typedef optional type;
0024
0025         template <class S, class Pos>
0026         struct apply :
0027           boost::mpl::if_<
0028             is_error<typename P::template apply<S, Pos> >,
0029             accept<Default, S, Pos>,
0030             // is_error evaluates it anyway
0031             typename P::template apply<S, Pos>::type
0032           >
0033         {};
0034       };
0035     }
0036   }
0037 }
0038
0039 #endif
0040