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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<ArgTypes...>>::type {}; | |
23 | |
24 // ArgsAreConvertible<CheckedType, ArgTypes...>::value is true iff all types in | |
25 // ArgTypes are convertible to CheckedType. | |
26 template <class...> | |
27 struct ArgsAreConvertible : std::true_type {}; | |
28 template <class CheckedType, class FirstArgType, class... ArgTypes> | |
29 struct ArgsAreConvertible<CheckedType, FirstArgType, ArgTypes...> | |
30 : std::conditional<std::is_convertible<FirstArgType, CheckedType>::value, | |
31 ArgsAreConvertible<CheckedType, ArgTypes...>, | |
32 std::false_type>::type {}; | |
33 | |
34 // When the following call is made: | |
35 // GetValueFromArgList(CallFirstTag(), GetterType(), args...); | |
36 // The compiler prefers using the first overload of GetValueFromArgList(), | |
37 // because the type of the first argument matches exactly. This overload returns | |
38 // getter.GetValueFromArg(first_arg), where |first_arg| is the first element in | |
39 // |args|. If getter.GetValueFromArg(first_arg) can't be called, the compiler | |
40 // uses the second or the third overload instead (depending on whether |args| is | |
41 // empty). The second overload discards |first_arg| and makes a recursive call | |
42 // to GetValueFromArgList() with CallFirstTag() as first argument. The third | |
43 // overload returns getter.GetDefaultValue(). GetValueFromArgList() generates a | |
44 // compile-time error if more than one of its arguments match the type of the | |
45 // argument of getter.GetValueFromArg(). | |
46 // | |
47 // |getter| must provide: | |
48 // | |
49 // ValueType: | |
50 // The return type of GetValueFromArgList(). | |
51 // | |
52 // ArgType: | |
53 // The type of the argument from which GetValueFromArgList() derives its | |
54 // return value. | |
55 // | |
56 // ValueType GetValueFromArg(ArgType): | |
57 // Converts an argument of type ArgType into a value returned by | |
58 // GetValueFromArgList(). | |
59 // | |
60 // ValueType GetDefaultValue(): | |
61 // Returns the value returned by GetValueFromArgList() if none of its | |
62 // arguments is of type ArgType. | |
63 | |
64 struct CallSecondTag {}; | |
65 struct CallFirstTag : CallSecondTag {}; | |
robliao
2017/04/26 19:45:54
To make sure I understand this correctly, these ar
fdoray
2017/04/26 20:25:14
Done.
| |
66 | |
67 // Overload 1: Get value from first argument. Check that no argument in |args| | |
68 // has the same type as |first_arg|. | |
69 template <class GetterType, | |
70 class FirstArgType, | |
71 class... ArgTypes, | |
72 class TestGetValueFromArgDefined = | |
73 decltype(std::declval<GetterType>().GetValueFromArg( | |
74 std::declval<FirstArgType>()))> | |
75 constexpr typename GetterType::ValueType GetValueFromArgList( | |
76 CallFirstTag, | |
77 GetterType getter, | |
78 const FirstArgType& first_arg, | |
79 const ArgTypes&... args) { | |
80 static_assert(!HasArgOfType<FirstArgType, ArgTypes...>::value, | |
81 "Multiple arguments of the same type were provided to the " | |
82 "constructor of TaskTraits."); | |
83 return getter.GetValueFromArg(first_arg); | |
84 } | |
85 | |
86 // Overload 2: Discard first argument. | |
87 template <class GetterType, class FirstArgType, class... ArgTypes> | |
88 constexpr typename GetterType::ValueType GetValueFromArgList( | |
89 CallSecondTag, | |
90 GetterType getter, | |
91 const FirstArgType&, | |
92 const ArgTypes&... args) { | |
93 return GetValueFromArgList(CallFirstTag(), getter, args...); | |
94 } | |
95 | |
96 // Overload 3: Default value. | |
97 template <class GetterType> | |
98 constexpr typename GetterType::ValueType GetValueFromArgList( | |
99 CallSecondTag, | |
100 GetterType getter) { | |
101 return getter.GetDefaultValue(); | |
102 } | |
103 | |
104 template <typename ArgType> | |
105 struct BooleanArgGetter { | |
106 using ValueType = bool; | |
107 constexpr ValueType GetValueFromArg(ArgType) const { return true; } | |
108 constexpr ValueType GetDefaultValue() const { return false; } | |
109 }; | |
110 | |
111 template <typename ArgType, ArgType DefaultValue> | |
112 struct EnumArgGetter { | |
113 using ValueType = ArgType; | |
114 constexpr ValueType GetValueFromArg(ArgType arg) const { return arg; } | |
115 constexpr ValueType GetDefaultValue() const { return DefaultValue; } | |
116 }; | |
117 | |
118 } // namespace internal | |
119 } // namespace base | |
120 | |
121 #endif // BASE_TASK_SCHEDULER_TASK_TRAITS_DETAILS_H_ | |
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