base/template_util.h

   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
  12 namespace base {
  13
  14 // template definitions from tr1
  15
  16 template<class T, T v>
  17 struct integral_constant {
  18   static const T value = v;
  19   typedef T value_type;
  20   typedef integral_constant<T, v> type;
  21 };
  22
  23 template <class T, T v> const T integral_constant<T, v>::value;
  24
  25 typedef integral_constant<bool, true> true_type;
  26 typedef integral_constant<bool, false> false_type;
  27
  28 template <class T> struct is_pointer : false_type {};
  29 template <class T> struct is_pointer<T*> : true_type {};
  30
  31 template <class T, class U> struct is_same : public false_type {};
  32 template <class T> struct is_same<T,T> : true_type {};
  33
  34 template<class> struct is_array : public false_type {};
  35 template<class T, size_t n> struct is_array<T[n]> : public true_type {};
  36 template<class T> struct is_array<T[]> : public true_type {};
  37
  38 template <class T> struct is_non_const_reference : false_type {};
  39 template <class T> struct is_non_const_reference<T&> : true_type {};
  40 template <class T> struct is_non_const_reference<const T&> : false_type {};
  41
  42 template <class T> struct is_const : false_type {};
  43 template <class T> struct is_const<const T> : true_type {};
  44
  45 template <class T> struct is_void : false_type {};
  46 template <> struct is_void<void> : true_type {};
  47
  48 namespace internal {
  49
  50 // Types YesType and NoType are guaranteed such that sizeof(YesType) <
  51 // sizeof(NoType).
  52 typedef char YesType;
  53
  54 struct 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
  67 struct 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.
  80 struct 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.
  94 template <typename From, typename To>
  95 struct is_convertible
  96     : integral_constant<bool,
  97                         sizeof(internal::ConvertHelper::Test<To>(
  98                                    internal::ConvertHelper::Create<From>())) ==
  99                         sizeof(internal::YesType)> {
 100 };
 101
 102 template <typename T>
 103 struct is_class
 104     : integral_constant<bool,
 105                         sizeof(internal::IsClassHelper::Test<T>(0)) ==
 106                             sizeof(internal::YesType)> {
 107 };
 108
 109 template<bool B, class T = void>
 110 struct enable_if {};
 111
 112 template<class T>
 113 struct enable_if<true, T> { typedef T type; };
 114
 115 }  // namespace base
 116
 117 #endif  // BASE_TEMPLATE_UTIL_H_