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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 // The compiler prefers using the first overload of GetValueFromArgListImpl(), | |
27 // because the type of the first argument matches exactly. This overload returns | |
28 // getter.GetValueFromArg(first_arg), where |first_arg| is the first element in | |
29 // |args|. If getter.GetValueFromArg(first_arg) isn't defined, the compiler uses | |
30 // the second or the third overload instead (depending on whether |args| is | |
31 // empty). The second overload discards |first_arg| and makes a recursive call | |
32 // to GetValueFromArgListImpl() with CallFirstTag() as first argument. The third | |
33 // overload returns getter.GetDefaultValue(). | |
34 | |
35 // Tag dispatching. | |
36 struct CallSecondTag {}; | |
37 struct CallFirstTag : CallSecondTag {}; | |
38 | |
39 // Overload 1: Get value from first argument. Check that no argument in |args| | |
40 // has the same type as |first_arg|. | |
41 template <class GetterType, | |
42 class FirstArgType, | |
43 class... ArgTypes, | |
44 class TestGetValueFromArgDefined = | |
45 decltype(std::declval<GetterType>().GetValueFromArg( | |
46 std::declval<FirstArgType>()))> | |
47 constexpr typename GetterType::ValueType GetValueFromArgListImpl( | |
48 CallFirstTag, | |
49 GetterType getter, | |
50 const FirstArgType& first_arg, | |
51 const ArgTypes&... args) { | |
52 static_assert(!HasArgOfType<FirstArgType, ArgTypes...>::value, | |
53 "Multiple arguments of the same type were provided to the " | |
54 "constructor of TaskTraits."); | |
55 return getter.GetValueFromArg(first_arg); | |
56 } | |
57 | |
58 // Overload 2: Discard first argument. | |
59 template <class GetterType, class FirstArgType, class... ArgTypes> | |
60 constexpr typename GetterType::ValueType GetValueFromArgListImpl( | |
61 CallSecondTag, | |
62 GetterType getter, | |
63 const FirstArgType&, | |
64 const ArgTypes&... args) { | |
65 return GetValueFromArgListImpl(CallFirstTag(), getter, args...); | |
66 } | |
67 | |
68 // Overload 3: Default value. | |
69 template <class GetterType> | |
70 constexpr typename GetterType::ValueType GetValueFromArgListImpl( | |
71 CallFirstTag, | |
gab
2017/04/28 19:00:31
I meant to reorder this to be Overload 1 so that l
fdoray
2017/05/01 17:13:10
Done.
| |
72 GetterType getter) { | |
73 return getter.GetDefaultValue(); | |
74 } | |
75 | |
76 // If there is an argument |arg_of_type| of type Getter::ArgType in |args|, | |
77 // returns getter.GetValueFromArg(arg_of_type). If there are more than one | |
78 // argument of type Getter::ArgType in |args|, generates a compile-time error. | |
79 // Otherwise, returns getter.GetDefaultValue(). | |
80 // | |
81 // |getter| must provide: | |
82 // | |
83 // ValueType: | |
84 // The return type of GetValueFromArgListImpl(). | |
85 // | |
86 // ArgType: | |
87 // The type of the argument from which GetValueFromArgListImpl() derives its | |
88 // return value. | |
89 // | |
90 // ValueType GetValueFromArg(ArgType): | |
91 // Converts an argument of type ArgType into a value returned by | |
92 // GetValueFromArgListImpl(). | |
93 // | |
94 // ValueType GetDefaultValue(): | |
95 // Returns the value returned by GetValueFromArgListImpl() if none of its | |
96 // arguments is of type ArgType. | |
97 template <class GetterType, class... ArgTypes> | |
98 constexpr typename GetterType::ValueType GetValueFromArgList( | |
99 GetterType getter, | |
100 const ArgTypes&... args) { | |
101 return GetValueFromArgListImpl(CallFirstTag(), getter, args...); | |
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 // Allows instantiation of multiple types in one statement. Used to prevent | |
119 // instantiation of the constructor of TaskTraits with inappropriate argument | |
120 // types. | |
121 template <class...> | |
122 struct InitTypes {}; | |
123 | |
124 } // namespace internal | |
125 } // namespace base | |
126 | |
127 #endif // BASE_TASK_SCHEDULER_TASK_TRAITS_DETAILS_H_ | |
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