// Copyright 2017 The Abseil Authors. // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // https://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. #ifndef ABSL_BASE_INTERNAL_INLINE_VARIABLE_H_ #define ABSL_BASE_INTERNAL_INLINE_VARIABLE_H_ #include <type_traits> #include "absl/base/internal/identity.h" // File: // This file define a macro that allows the creation of or emulation of C++17 // inline variables based on whether or not the feature is supported. //////////////////////////////////////////////////////////////////////////////// // Macro: ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) // // Description: // Expands to the equivalent of an inline constexpr instance of the specified // `type` and `name`, initialized to the value `init`. If the compiler being // used is detected as supporting actual inline variables as a language // feature, then the macro expands to an actual inline variable definition. // // Requires: // `type` is a type that is usable in an extern variable declaration. // // Requires: `name` is a valid identifier // // Requires: // `init` is an expression that can be used in the following definition: // constexpr type name = init; // // Usage: // // // Equivalent to: `inline constexpr size_t variant_npos = -1;` // ABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, -1); // // Differences in implementation: // For a direct, language-level inline variable, decltype(name) will be the // type that was specified along with const qualification, whereas for // emulated inline variables, decltype(name) may be different (in practice // it will likely be a reference type). //////////////////////////////////////////////////////////////////////////////// #ifdef __cpp_inline_variables // Clang's -Wmissing-variable-declarations option erroneously warned that // inline constexpr objects need to be pre-declared. This has now been fixed, // but we will need to support this workaround for people building with older // versions of clang. // // Bug: https://bugs.llvm.org/show_bug.cgi?id=35862 // // Note: // type_identity_t is used here so that the const and name are in the // appropriate place for pointer types, reference types, function pointer // types, etc.. #if defined(__clang__) #define ABSL_INTERNAL_EXTERN_DECL(type, name) … #else // Otherwise, just define the macro to do nothing. #define ABSL_INTERNAL_EXTERN_DECL … #endif // defined(__clang__) // See above comment at top of file for details. #define ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) … #else // See above comment at top of file for details. // // Note: // type_identity_t is used here so that the const and name are in the // appropriate place for pointer types, reference types, function pointer // types, etc.. #define ABSL_INTERNAL_INLINE_CONSTEXPR … #endif // __cpp_inline_variables #endif // ABSL_BASE_INTERNAL_INLINE_VARIABLE_H_