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 // Member function pointer detection. This is built-in to C++ 11's stdlib, and
  32 // we can remove this when we switch to it.
  33 template<typename T>
  34 struct is_member_function_pointer : false_type {};
  35
  36 template <typename R, typename Z, typename... A>
  37 struct is_member_function_pointer<R(Z::*)(A...)> : true_type {};
  38 template <typename R, typename Z, typename... A>
  39 struct is_member_function_pointer<R(Z::*)(A...) const> : true_type {};
  40
  41
  42 template <class T, class U> struct is_same : public false_type {};
  43 template <class T> struct is_same<T,T> : true_type {};
  44
  45 template<class> struct is_array : public false_type {};
  46 template<class T, size_t n> struct is_array<T[n]> : public true_type {};
  47 template<class T> struct is_array<T[]> : public true_type {};
  48
  49 template <class T> struct is_non_const_reference : false_type {};
  50 template <class T> struct is_non_const_reference<T&> : true_type {};
  51 template <class T> struct is_non_const_reference<const T&> : false_type {};
  52
  53 template <class T> struct is_const : false_type {};
  54 template <class T> struct is_const<const T> : true_type {};
  55
  56 template <class T> struct is_void : false_type {};
  57 template <> struct is_void<void> : true_type {};
  58
  59 namespace internal {
  60
  61 // Types YesType and NoType are guaranteed such that sizeof(YesType) <
  62 // sizeof(NoType).
  63 typedef char YesType;
  64
  65 struct NoType {
  66   YesType dummy[2];
  67 };
  68
  69 // This class is an implementation detail for is_convertible, and you
  70 // don't need to know how it works to use is_convertible. For those
  71 // who care: we declare two different functions, one whose argument is
  72 // of type To and one with a variadic argument list. We give them
  73 // return types of different size, so we can use sizeof to trick the
  74 // compiler into telling us which function it would have chosen if we
  75 // had called it with an argument of type From.  See Alexandrescu's
  76 // _Modern C++ Design_ for more details on this sort of trick.
  77
  78 struct ConvertHelper {
  79   template <typename To>
  80   static YesType Test(To);
  81
  82   template <typename To>
  83   static NoType Test(...);
  84
  85   template <typename From>
  86   static From& Create();
  87 };
  88
  89 // Used to determine if a type is a struct/union/class. Inspired by Boost's
  90 // is_class type_trait implementation.
  91 struct IsClassHelper {
  92   template <typename C>
  93   static YesType Test(void(C::*)(void));
  94
  95   template <typename C>
  96   static NoType Test(...);
  97 };
  98
  99 }  // namespace internal
 100
 101 // Inherits from true_type if From is convertible to To, false_type otherwise.
 102 //
 103 // Note that if the type is convertible, this will be a true_type REGARDLESS
 104 // of whether or not the conversion would emit a warning.
 105 template <typename From, typename To>
 106 struct is_convertible
 107     : integral_constant<bool,
 108                         sizeof(internal::ConvertHelper::Test<To>(
 109                                    internal::ConvertHelper::Create<From>())) ==
 110                         sizeof(internal::YesType)> {
 111 };
 112
 113 template <typename T>
 114 struct is_class
 115     : integral_constant<bool,
 116                         sizeof(internal::IsClassHelper::Test<T>(0)) ==
 117                             sizeof(internal::YesType)> {
 118 };
 119
 120 template<bool B, class T = void>
 121 struct enable_if {};
 122
 123 template<class T>
 124 struct enable_if<true, T> { typedef T type; };
 125
 126 }  // namespace base
 127
 128 #endif  // BASE_TEMPLATE_UTIL_H_