inline_variable.h
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1 // Copyright 2017 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 
15 #ifndef ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
16 #define ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
17 
18 #include <type_traits>
19 
21 
22 // File:
23 // This file define a macro that allows the creation of or emulation of C++17
24 // inline variables based on whether or not the feature is supported.
25 
27 // Macro: ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init)
28 //
29 // Description:
30 // Expands to the equivalent of an inline constexpr instance of the specified
31 // `type` and `name`, initialized to the value `init`. If the compiler being
32 // used is detected as supporting actual inline variables as a language
33 // feature, then the macro expands to an actual inline variable definition.
34 //
35 // Requires:
36 // `type` is a type that is usable in an extern variable declaration.
37 //
38 // Requires: `name` is a valid identifier
39 //
40 // Requires:
41 // `init` is an expression that can be used in the following definition:
42 // constexpr type name = init;
43 //
44 // Usage:
45 //
46 // // Equivalent to: `inline constexpr size_t variant_npos = -1;`
47 // ABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, -1);
48 //
49 // Differences in implementation:
50 // For a direct, language-level inline variable, decltype(name) will be the
51 // type that was specified along with const qualification, whereas for
52 // emulated inline variables, decltype(name) may be different (in practice
53 // it will likely be a reference type).
55 
56 #ifdef __cpp_inline_variables
57 
58 // Clang's -Wmissing-variable-declarations option erroneously warned that
59 // inline constexpr objects need to be pre-declared. This has now been fixed,
60 // but we will need to support this workaround for people building with older
61 // versions of clang.
62 //
63 // Bug: https://bugs.llvm.org/show_bug.cgi?id=35862
64 //
65 // Note:
66 // identity_t is used here so that the const and name are in the
67 // appropriate place for pointer types, reference types, function pointer
68 // types, etc..
69 #if defined(__clang__)
70 #define ABSL_INTERNAL_EXTERN_DECL(type, name) \
71  extern const ::absl::internal::identity_t<type> name;
72 #else // Otherwise, just define the macro to do nothing.
73 #define ABSL_INTERNAL_EXTERN_DECL(type, name)
74 #endif // defined(__clang__)
75 
76 // See above comment at top of file for details.
77 #define ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) \
78  ABSL_INTERNAL_EXTERN_DECL(type, name) \
79  inline constexpr ::absl::internal::identity_t<type> name = init
80 
81 #else
82 
83 // See above comment at top of file for details.
84 //
85 // Note:
86 // identity_t is used here so that the const and name are in the
87 // appropriate place for pointer types, reference types, function pointer
88 // types, etc..
89 #define ABSL_INTERNAL_INLINE_CONSTEXPR(var_type, name, init) \
90  template <class /*AbslInternalDummy*/ = void> \
91  struct AbslInternalInlineVariableHolder##name { \
92  static constexpr ::absl::internal::identity_t<var_type> kInstance = init; \
93  }; \
94  \
95  template <class AbslInternalDummy> \
96  constexpr ::absl::internal::identity_t<var_type> \
97  AbslInternalInlineVariableHolder##name<AbslInternalDummy>::kInstance; \
98  \
99  static constexpr const ::absl::internal::identity_t<var_type>& \
100  name = /* NOLINT */ \
101  AbslInternalInlineVariableHolder##name<>::kInstance; \
102  static_assert(sizeof(void (*)(decltype(name))) != 0, \
103  "Silence unused variable warnings.")
104 
105 #endif // __cpp_inline_variables
106 
107 #endif // ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_


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autogenerated on Tue Jun 18 2019 19:44:36