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 20 #include "absl/base/internal/identity.h" 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 26 //////////////////////////////////////////////////////////////////////////////// 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). 54 //////////////////////////////////////////////////////////////////////////////// 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_ 108