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