1 // Copyright 2017 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_TASK_SCHEDULER_TASK_TRAITS_DETAILS_H_ 6 #define BASE_TASK_SCHEDULER_TASK_TRAITS_DETAILS_H_ 7 8 #include <type_traits> 9 #include <utility> 10 11 namespace base { 12 namespace internal { 13 14 // HasArgOfType<CheckedType, ArgTypes...>::value is true iff a type in ArgTypes 15 // matches CheckedType. 16 template <class...> 17 struct HasArgOfType : std::false_type {}; 18 template <class CheckedType, class FirstArgType, class... ArgTypes> 19 struct HasArgOfType<CheckedType, FirstArgType, ArgTypes...> 20 : std::conditional<std::is_same<CheckedType, FirstArgType>::value, 21 std::true_type, 22 HasArgOfType<CheckedType, ArgTypes...>>::type {}; 23 24 // When the following call is made: 25 // GetValueFromArgListImpl(CallFirstTag(), GetterType(), args...); 26 // If |args| is empty, the compiler selects the first overload. This overload 27 // returns getter.GetDefaultValue(). If |args| is not empty, the compiler 28 // prefers using the second overload because the type of the first argument 29 // matches exactly. This overload returns getter.GetValueFromArg(first_arg), 30 // where |first_arg| is the first element in |args|. If 31 // getter.GetValueFromArg(first_arg) isn't defined, the compiler uses the third 32 // overload instead. This overload discards the first argument in |args| and 33 // makes a recursive call to GetValueFromArgListImpl() with CallFirstTag() as 34 // first argument. 35 36 // Tag dispatching. 37 struct CallSecondTag {}; 38 struct CallFirstTag : CallSecondTag {}; 39 40 // Overload 1: Default value. 41 template <class GetterType> 42 constexpr typename GetterType::ValueType GetValueFromArgListImpl( 43 CallFirstTag, 44 GetterType getter) { 45 return getter.GetDefaultValue(); 46 } 47 48 // Overload 2: Get value from first argument. Check that no argument in |args| 49 // has the same type as |first_arg|. 50 template <class GetterType, 51 class FirstArgType, 52 class... ArgTypes, 53 class TestGetValueFromArgDefined = 54 decltype(std::declval<GetterType>().GetValueFromArg( 55 std::declval<FirstArgType>()))> 56 constexpr typename GetterType::ValueType GetValueFromArgListImpl( 57 CallFirstTag, 58 GetterType getter, 59 const FirstArgType& first_arg, 60 const ArgTypes&... args) { 61 static_assert(!HasArgOfType<FirstArgType, ArgTypes...>::value, 62 "Multiple arguments of the same type were provided to the " 63 "constructor of TaskTraits."); 64 return getter.GetValueFromArg(first_arg); 65 } 66 67 // Overload 3: Discard first argument. 68 template <class GetterType, class FirstArgType, class... ArgTypes> 69 constexpr typename GetterType::ValueType GetValueFromArgListImpl( 70 CallSecondTag, 71 GetterType getter, 72 const FirstArgType&, 73 const ArgTypes&... args) { 74 return GetValueFromArgListImpl(CallFirstTag(), getter, args...); 75 } 76 77 // If there is an argument |arg_of_type| of type Getter::ArgType in |args|, 78 // returns getter.GetValueFromArg(arg_of_type). If there are more than one 79 // argument of type Getter::ArgType in |args|, generates a compile-time error. 80 // Otherwise, returns getter.GetDefaultValue(). 81 // 82 // |getter| must provide: 83 // 84 // ValueType: 85 // The return type of GetValueFromArgListImpl(). 86 // 87 // ArgType: 88 // The type of the argument from which GetValueFromArgListImpl() derives its 89 // return value. 90 // 91 // ValueType GetValueFromArg(ArgType): 92 // Converts an argument of type ArgType into a value returned by 93 // GetValueFromArgListImpl(). 94 // 95 // ValueType GetDefaultValue(): 96 // Returns the value returned by GetValueFromArgListImpl() if none of its 97 // arguments is of type ArgType. 98 template <class GetterType, class... ArgTypes> 99 constexpr typename GetterType::ValueType GetValueFromArgList( 100 GetterType getter, 101 const ArgTypes&... args) { 102 return GetValueFromArgListImpl(CallFirstTag(), getter, args...); 103 } 104 105 template <typename ArgType> 106 struct BooleanArgGetter { 107 using ValueType = bool; 108 constexpr ValueType GetValueFromArg(ArgType) const { return true; } 109 constexpr ValueType GetDefaultValue() const { return false; } 110 }; 111 112 template <typename ArgType, ArgType DefaultValue> 113 struct EnumArgGetter { 114 using ValueType = ArgType; 115 constexpr ValueType GetValueFromArg(ArgType arg) const { return arg; } 116 constexpr ValueType GetDefaultValue() const { return DefaultValue; } 117 }; 118 119 // Allows instantiation of multiple types in one statement. Used to prevent 120 // instantiation of the constructor of TaskTraits with inappropriate argument 121 // types. 122 template <class...> 123 struct InitTypes {}; 124 125 } // namespace internal 126 } // namespace base 127 128 #endif // BASE_TASK_SCHEDULER_TASK_TRAITS_DETAILS_H_ 129