1 #ifndef BOOST_METAPARSE_V1_OPTIONAL_HPP 2 #define BOOST_METAPARSE_V1_OPTIONAL_HPP 3 4 // Copyright Abel Sinkovics (abel@sinkovics.hu) 2015. 5 // Distributed under the Boost Software License, Version 1.0. 6 // (See accompanying file LICENSE_1_0.txt or copy at 7 // http://www.boost.org/LICENSE_1_0.txt) 8 9 #include <boost/metaparse/v1/is_error.hpp> 10 #include <boost/metaparse/v1/accept.hpp> 11 12 #include <boost/mpl/if.hpp> 13 14 namespace boost 15 { 16 namespace metaparse 17 { 18 namespace v1 19 { 20 template <class P, class Default = void> 21 struct optional 22 { 23 typedef optional type; 24 25 template <class S, class Pos> 26 struct apply : 27 boost::mpl::if_< 28 is_error<typename P::template apply<S, Pos> >, 29 accept<Default, S, Pos>, 30 // is_error evaluates it anyway 31 typename P::template apply<S, Pos>::type 32 > 33 {}; 34 }; 35 } 36 } 37 } 38 39 #endif 40 41