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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 // 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 // GetValueFromArgListImpl(CallFirstTag(), GetterType(), args...); | |
36 // The compiler prefers using the first overload of GetValueFromArgListImpl(), | |
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) isn't defined, the compiler uses | |
40 // 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 GetValueFromArgListImpl() with CallFirstTag() as first argument. The third | |
43 // overload returns getter.GetDefaultValue(). | |
44 | |
45 // Tag dispatching. | |
46 struct CallSecondTag {}; | |
47 struct CallFirstTag : CallSecondTag {}; | |
48 | |
49 // Overload 1: Get value from first argument. Check that no argument in |args| | |
50 // has the same type as |first_arg|. | |
51 template <class GetterType, | |
52 class FirstArgType, | |
53 class... ArgTypes, | |
54 class TestGetValueFromArgDefined = | |
55 decltype(std::declval<GetterType>().GetValueFromArg( | |
56 std::declval<FirstArgType>()))> | |
57 constexpr typename GetterType::ValueType GetValueFromArgListImpl( | |
58 CallFirstTag, | |
59 GetterType getter, | |
60 const FirstArgType& first_arg, | |
61 const ArgTypes&... args) { | |
62 static_assert(!HasArgOfType<FirstArgType, ArgTypes...>::value, | |
63 "Multiple arguments of the same type were provided to the " | |
64 "constructor of TaskTraits."); | |
65 return getter.GetValueFromArg(first_arg); | |
66 } | |
67 | |
68 // Overload 2: Discard first argument. | |
69 template <class GetterType, class FirstArgType, class... ArgTypes> | |
70 constexpr typename GetterType::ValueType GetValueFromArgListImpl( | |
71 CallSecondTag, | |
72 GetterType getter, | |
73 const FirstArgType&, | |
74 const ArgTypes&... args) { | |
75 return GetValueFromArgListImpl(CallFirstTag(), getter, args...); | |
76 } | |
77 | |
78 // Overload 3: Default value. | |
79 template <class GetterType> | |
80 constexpr typename GetterType::ValueType GetValueFromArgListImpl( | |
81 CallSecondTag, | |
gab
2017/04/28 18:03:40
I guess it doesn't matter that this is CallFirstTa
fdoray
2017/04/28 18:40:36
Done.
| |
82 GetterType getter) { | |
83 return getter.GetDefaultValue(); | |
84 } | |
85 | |
86 // If there is an argument |arg_of_type| of type Getter::ArgType in |args|, | |
87 // returns getter.GetValueFromArg(arg_of_type). If there are more than one | |
88 // argument of type Getter::ArgType in |args|, generates a compile-time error. | |
89 // Otherwise, returns getter.GetDefaultValue(). | |
90 // | |
91 // |getter| must provide: | |
92 // | |
93 // ValueType: | |
94 // The return type of GetValueFromArgListImpl(). | |
95 // | |
96 // ArgType: | |
97 // The type of the argument from which GetValueFromArgListImpl() derives its | |
98 // return value. | |
99 // | |
100 // ValueType GetValueFromArg(ArgType): | |
101 // Converts an argument of type ArgType into a value returned by | |
102 // GetValueFromArgListImpl(). | |
103 // | |
104 // ValueType GetDefaultValue(): | |
105 // Returns the value returned by GetValueFromArgListImpl() if none of its | |
106 // arguments is of type ArgType. | |
107 template <class GetterType, class... ArgTypes> | |
108 constexpr typename GetterType::ValueType GetValueFromArgList( | |
109 GetterType getter, | |
110 const ArgTypes&... args) { | |
111 return GetValueFromArgListImpl(CallFirstTag(), getter, args...); | |
112 } | |
113 | |
114 template <typename ArgType> | |
115 struct BooleanArgGetter { | |
116 using ValueType = bool; | |
117 constexpr ValueType GetValueFromArg(ArgType) const { return true; } | |
118 constexpr ValueType GetDefaultValue() const { return false; } | |
119 }; | |
120 | |
121 template <typename ArgType, ArgType DefaultValue> | |
122 struct EnumArgGetter { | |
123 using ValueType = ArgType; | |
124 constexpr ValueType GetValueFromArg(ArgType arg) const { return arg; } | |
125 constexpr ValueType GetDefaultValue() const { return DefaultValue; } | |
126 }; | |
127 | |
128 } // namespace internal | |
129 } // namespace base | |
130 | |
131 #endif // BASE_TASK_SCHEDULER_TASK_TRAITS_DETAILS_H_ | |
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