template_util.h revision 8bcbed890bc3ce4d7a057a8f32cab53fa534672e
1// Copyright (c) 2011 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef BASE_TEMPLATE_UTIL_H_
6#define BASE_TEMPLATE_UTIL_H_
7
8#include <cstddef>  // For size_t.
9
10#include "build/build_config.h"
11
12namespace base {
13
14// template definitions from tr1
15
16template<class T, T v>
17struct integral_constant {
18  static const T value = v;
19  typedef T value_type;
20  typedef integral_constant<T, v> type;
21};
22
23template <class T, T v> const T integral_constant<T, v>::value;
24
25typedef integral_constant<bool, true> true_type;
26typedef integral_constant<bool, false> false_type;
27
28template <class T> struct is_pointer : false_type {};
29template <class T> struct is_pointer<T*> : true_type {};
30
31template <class T, class U> struct is_same : public false_type {};
32template <class T> struct is_same<T,T> : true_type {};
33
34template<class> struct is_array : public false_type {};
35template<class T, size_t n> struct is_array<T[n]> : public true_type {};
36template<class T> struct is_array<T[]> : public true_type {};
37
38template <class T> struct is_non_const_reference : false_type {};
39template <class T> struct is_non_const_reference<T&> : true_type {};
40template <class T> struct is_non_const_reference<const T&> : false_type {};
41
42template <class T> struct is_const : false_type {};
43template <class T> struct is_const<const T> : true_type {};
44
45template <class T> struct is_void : false_type {};
46template <> struct is_void<void> : true_type {};
47
48namespace internal {
49
50// Types YesType and NoType are guaranteed such that sizeof(YesType) <
51// sizeof(NoType).
52typedef char YesType;
53
54struct NoType {
55  YesType dummy[2];
56};
57
58// This class is an implementation detail for is_convertible, and you
59// don't need to know how it works to use is_convertible. For those
60// who care: we declare two different functions, one whose argument is
61// of type To and one with a variadic argument list. We give them
62// return types of different size, so we can use sizeof to trick the
63// compiler into telling us which function it would have chosen if we
64// had called it with an argument of type From.  See Alexandrescu's
65// _Modern C++ Design_ for more details on this sort of trick.
66
67struct ConvertHelper {
68  template <typename To>
69  static YesType Test(To);
70
71  template <typename To>
72  static NoType Test(...);
73
74  template <typename From>
75  static From& Create();
76};
77
78// Used to determine if a type is a struct/union/class. Inspired by Boost's
79// is_class type_trait implementation.
80struct IsClassHelper {
81  template <typename C>
82  static YesType Test(void(C::*)(void));
83
84  template <typename C>
85  static NoType Test(...);
86};
87
88}  // namespace internal
89
90// Inherits from true_type if From is convertible to To, false_type otherwise.
91//
92// Note that if the type is convertible, this will be a true_type REGARDLESS
93// of whether or not the conversion would emit a warning.
94template <typename From, typename To>
95struct is_convertible
96    : integral_constant<bool,
97                        sizeof(internal::ConvertHelper::Test<To>(
98                                   internal::ConvertHelper::Create<From>())) ==
99                        sizeof(internal::YesType)> {
100};
101
102template <typename T>
103struct is_class
104    : integral_constant<bool,
105                        sizeof(internal::IsClassHelper::Test<T>(0)) ==
106                            sizeof(internal::YesType)> {
107};
108
109template<bool B, class T = void>
110struct enable_if {};
111
112template<class T>
113struct enable_if<true, T> { typedef T type; };
114
115}  // namespace base
116
117#endif  // BASE_TEMPLATE_UTIL_H_
118