source: NonGTP/Boost/boost/pending/ct_if.hpp @ 857

Revision 857, 3.2 KB checked in by igarcia, 19 years ago (diff)
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[857]1// (C) Copyright Jeremy Siek 2000.
2// Distributed under the Boost Software License, Version 1.0. (See
3// accompanying file LICENSE_1_0.txt or copy at
4// http://www.boost.org/LICENSE_1_0.txt)
5
6// The ct_if implementation that avoids partial specialization is
7// based on the IF class by Ulrich W. Eisenecker and Krzysztof
8// Czarnecki.
9
10#ifndef BOOST_CT_IF_HPP
11#define BOOST_CT_IF_HPP
12
13#include <boost/config.hpp>
14
15/*
16  There is a bug in the Borland compiler with regards to using
17  integers to specialize templates. This made it hard to use ct_if in
18  the graph library. Changing from 'ct_if' to 'ct_if_t' fixed the
19  problem.
20*/
21
22#include <boost/type_traits/integral_constant.hpp> // true_type and false_type
23
24namespace boost {
25
26  struct ct_if_error { };
27
28  template <class A, class B>
29  struct ct_and { typedef false_type type; };
30  template <> struct ct_and<true_type,true_type> { typedef true_type type; };
31
32  template <class A> struct ct_not { typedef ct_if_error type; };
33  template <> struct ct_not<true_type> { typedef false_type type; };
34  template <> struct ct_not<false_type> { typedef true_type type; };
35
36#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
37
38// agurt, 15/sep/02: in certain cases Borland has problems with
39// choosing the right 'ct_if' specialization even though 'cond'
40// _does_ equal '1'; the easiest way to fix it is to make first
41// 'ct_if' non-type template parameter boolean.
42#if !defined(__BORLANDC__)
43  template <bool cond, class A, class B>
44  struct ct_if { typedef ct_if_error type; };
45  template <class A, class B>
46  struct ct_if<true, A, B> { typedef A type; };
47  template <class A, class B>
48  struct ct_if<false, A, B> { typedef B type; };
49#else
50  template <bool cond, class A, class B>
51  struct ct_if { typedef A type; };
52  template <class A, class B>
53  struct ct_if<false, A, B> { typedef B type; };
54#endif
55
56  template <class cond, class A, class B>
57  struct ct_if_t { typedef ct_if_error type; };
58  template <class A, class B>
59  struct ct_if_t<true_type, A, B> { typedef A type; };
60  template <class A, class B>
61  struct ct_if_t<false_type, A, B> { typedef B type; };
62
63#else
64
65  namespace detail {
66
67    template <int condition, class A, class B> struct IF;
68    template <int condition> struct SlectSelector;
69    struct SelectFirstType;
70    struct SelectSecondType;
71   
72    struct SelectFirstType {
73      template<class A, class B>
74      struct Template {        typedef A type; };
75    };
76   
77    struct SelectSecondType {
78      template<class A, class B>
79      struct Template { typedef B type; };
80    };
81   
82    template<int condition>
83    struct SlectSelector {
84      typedef SelectFirstType type;
85    };
86   
87    template <>
88    struct SlectSelector<0> {
89      typedef SelectSecondType type;
90    };
91
92  } // namespace detail
93   
94  template<int condition, class A, class B>
95  struct ct_if
96  {
97    typedef typename detail::SlectSelector<condition>::type Selector;
98    typedef typename Selector::template Template<A, B>::type type;
99  };
100 
101  template <class cond, class A, class B>
102  struct ct_if_t {
103    typedef typename ct_if<cond::value, A, B>::type type;
104  };
105
106#endif
107
108} // namespace boost
109
110#endif // BOOST_CT_IF_HPP
111
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