1 ///////////////////////////////////////////////////////////////////////////////
2 // external_accumulator.hpp
3 //
4 //  Copyright 2005 Eric Niebler. Distributed under the Boost
5 //  Software License, Version 1.0. (See accompanying file
6 //  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7 
8 #ifndef BOOST_ACCUMULATORS_FRAMEWORK_ACCUMULATORS_EXTERNAL_ACCUMULATOR_HPP_EAN_01_12_2005
9 #define BOOST_ACCUMULATORS_FRAMEWORK_ACCUMULATORS_EXTERNAL_ACCUMULATOR_HPP_EAN_01_12_2005
10 
11 #include <boost/mpl/placeholders.hpp>
12 #include <boost/parameter/keyword.hpp>
13 #include <boost/accumulators/framework/extractor.hpp>
14 #include <boost/accumulators/framework/depends_on.hpp> // for feature_tag
15 #include <boost/accumulators/framework/accumulator_base.hpp>
16 #include <boost/accumulators/framework/accumulators/reference_accumulator.hpp>
17 
18 namespace boost { namespace accumulators { namespace impl
19 {
20 
21     //////////////////////////////////////////////////////////////////////////
22     // external_impl
23     /// INTERNAL ONLY
24     ///
25     template<typename Accumulator, typename Tag>
26     struct external_impl
27       : accumulator_base
28     {
29         typedef typename Accumulator::result_type result_type;
30         typedef typename detail::feature_tag<Accumulator>::type feature_tag;
31 
external_implboost::accumulators::impl::external_impl32         external_impl(dont_care) {}
33 
34         template<typename Args>
resultboost::accumulators::impl::external_impl35         result_type result(Args const &args) const
36         {
37             return this->extract_(args, args[parameter::keyword<Tag>::instance | 0]);
38         }
39 
40     private:
41 
42         template<typename Args>
extract_boost::accumulators::impl::external_impl43         static result_type extract_(Args const &args, int)
44         {
45             // No named parameter passed to the extractor. Maybe the external
46             // feature is held by reference<>.
47             extractor<feature_tag> extract;
48             return extract(accumulators::reference_tag<Tag>(args));
49         }
50 
51         template<typename Args, typename AccumulatorSet>
extract_boost::accumulators::impl::external_impl52         static result_type extract_(Args const &, AccumulatorSet const &acc)
53         {
54             // OK, a named parameter for this external feature was passed to the
55             // extractor, so use that.
56             extractor<feature_tag> extract;
57             return extract(acc);
58         }
59     };
60 
61 } // namespace impl
62 
63 namespace tag
64 {
65     //////////////////////////////////////////////////////////////////////////
66     // external
67     template<typename Feature, typename Tag, typename AccumulatorSet>
68     struct external
69       : depends_on<reference<AccumulatorSet, Tag> >
70     {
71         typedef
72             accumulators::impl::external_impl<
73                 detail::to_accumulator<Feature, mpl::_1, mpl::_2>
74               , Tag
75             >
76         impl;
77     };
78 
79     template<typename Feature, typename Tag>
80     struct external<Feature, Tag, void>
81       : depends_on<>
82     {
83         typedef
84             accumulators::impl::external_impl<
85                 detail::to_accumulator<Feature, mpl::_1, mpl::_2>
86               , Tag
87             >
88         impl;
89     };
90 }
91 
92 // for the purposes of feature-based dependency resolution,
93 // external_accumulator<Feature, Tag> provides the same feature as Feature
94 template<typename Feature, typename Tag, typename AccumulatorSet>
95 struct feature_of<tag::external<Feature, Tag, AccumulatorSet> >
96   : feature_of<Feature>
97 {
98 };
99 
100 // Note: Usually, the extractor is pulled into the accumulators namespace with
101 // a using directive, not the tag. But the external<> feature doesn't have an
102 // extractor, so we can put the external tag in the accumulators namespace
103 // without fear of a name conflict.
104 using tag::external;
105 
106 }} // namespace boost::accumulators
107 
108 #endif
109