1/*=============================================================================
2 Copyright (c) 2001-2014 Joel de Guzman
3 Copyright (c) 2001-2011 Hartmut Kaiser
4 http://spirit.sourceforge.net/
5
6 Distributed under the Boost Software License, Version 1.0. (See accompanying
7 file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
8=============================================================================*/
9#if !defined(BOOST_SPIRIT_X3_OPTIONAL_TRAITS_FEBRUARY_06_2007_1001AM)
10#define BOOST_SPIRIT_X3_OPTIONAL_TRAITS_FEBRUARY_06_2007_1001AM
11
12#include <boost/spirit/home/x3/support/unused.hpp>
13#include <boost/optional/optional.hpp>
14#include <boost/mpl/identity.hpp>
15
16namespace boost { namespace spirit { namespace x3 { namespace traits
17{
18 ///////////////////////////////////////////////////////////////////////////
19 template <typename T, typename Enable = void>
20 struct is_optional
21 : mpl::false_
22 {};
23
24 template <typename T>
25 struct is_optional<boost::optional<T>>
26 : mpl::true_
27 {};
28
29 ///////////////////////////////////////////////////////////////////////////
30 // build_optional
31 //
32 // Build a boost::optional from T. Return unused_type if T is unused_type.
33 ///////////////////////////////////////////////////////////////////////////
34 template <typename T>
35 struct build_optional
36 {
37 typedef boost::optional<T> type;
38 };
39
40 template <typename T>
41 struct build_optional<boost::optional<T> >
42 {
43 typedef boost::optional<T> type;
44 };
45
46 template <>
47 struct build_optional<unused_type>
48 {
49 typedef unused_type type;
50 };
51
52 ///////////////////////////////////////////////////////////////////////////
53 // optional_value
54 //
55 // Get the optional's value_type. Handles unused_type as well.
56 ///////////////////////////////////////////////////////////////////////////
57 template <typename T>
58 struct optional_value : mpl::identity<T> {};
59
60 template <typename T>
61 struct optional_value<boost::optional<T> >
62 : mpl::identity<T> {};
63
64 template <>
65 struct optional_value<unused_type>
66 : mpl::identity<unused_type> {};
67
68 template <>
69 struct optional_value<unused_type const>
70 : mpl::identity<unused_type> {};
71
72}}}}
73
74#endif
75

source code of boost/libs/spirit/include/boost/spirit/home/x3/support/traits/optional_traits.hpp