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