1// Copyright David Abrahams, Daniel Wallin 2003.
2// Distributed under the Boost Software License, Version 1.0.
3// (See accompanying file LICENSE_1_0.txt or copy at
4// http://www.boost.org/LICENSE_1_0.txt)
5
6#ifndef BOOST_PARAMETER_OPTIONAL_HPP
7#define BOOST_PARAMETER_OPTIONAL_HPP
8
9#include <boost/parameter/aux_/use_default.hpp>
10
11namespace boost { namespace parameter {
12
13 // This metafunction can be used to describe the treatment of particular
14 // named parameters for the purposes of overload elimination with SFINAE,
15 // by placing specializations in the parameters<...> list. In order for
16 // a treated function to participate in overload resolution:
17 //
18 // - The actual argument type matched by every keyword tag
19 // associated with a predicate must satisfy that predicate
20 //
21 // - If a keyword k is specified without an optional<...> or
22 // required<...> wrapper, it is treated as though
23 // optional<k> were specified.
24 template <
25 typename Tag
26 , typename Predicate = ::boost::parameter::aux::use_default
27 >
28 struct optional
29 {
30 typedef Tag key_type;
31 typedef Predicate predicate;
32 };
33}}
34
35#include <boost/parameter/config.hpp>
36
37#if defined(BOOST_PARAMETER_CAN_USE_MP11)
38#include <boost/mp11/integral.hpp>
39#else
40#include <boost/mpl/bool.hpp>
41#endif
42
43namespace boost { namespace parameter { namespace aux {
44
45 template <typename T>
46 struct is_optional
47#if defined(BOOST_PARAMETER_CAN_USE_MP11)
48 : ::boost::mp11::mp_false
49#else
50 : ::boost::mpl::false_
51#endif
52 {
53 };
54
55 template <typename Tag, typename Predicate>
56 struct is_optional< ::boost::parameter::optional<Tag,Predicate> >
57#if defined(BOOST_PARAMETER_CAN_USE_MP11)
58 : ::boost::mp11::mp_true
59#else
60 : ::boost::mpl::true_
61#endif
62 {
63 };
64}}} // namespace boost::parameter::aux
65
66#endif // include guard
67
68

source code of boost/libs/parameter/include/boost/parameter/optional.hpp