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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<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 | |
gab
2017/05/01 18:56:17
no "," before "because" IMO
fdoray
2017/05/01 19:29:30
Done.
| |
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_ | |
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