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Diffstat (limited to 'absl/base/internal/inline_variable.h')
| -rw-r--r-- | absl/base/internal/inline_variable.h | 127 |
1 files changed, 127 insertions, 0 deletions
diff --git a/absl/base/internal/inline_variable.h b/absl/base/internal/inline_variable.h new file mode 100644 index 000000000000..a65fe89354cb --- /dev/null +++ b/absl/base/internal/inline_variable.h @@ -0,0 +1,127 @@ +// 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 +// +// http://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_EMULATION_H_ +#define ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_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). +//////////////////////////////////////////////////////////////////////////////// + +// ABSL_INTERNAL_HAS_WARNING() +// +// If the compiler supports the `__has_warning` extension for detecting +// warnings, then this macro is defined to be `__has_warning`. +// +// If the compiler does not support `__has_warning`, invocations expand to 0. +// +// For clang's documentation of `__has_warning`, see +// https://clang.llvm.org/docs/LanguageExtensions.html#has-warning +#if defined(__has_warning) +#define ABSL_INTERNAL_HAS_WARNING __has_warning +#else // Otherwise, be optimistic and assume the warning is not enabled. +#define ABSL_INTERNAL_HAS_WARNING(warning) 0 +#endif // defined(__has_warning) + +// If the compiler supports inline variables and does not warn when used... +#if defined(__cpp_inline_variables) && \ + !ABSL_INTERNAL_HAS_WARNING("-Wc++98-c++11-c++14-compat") + +// 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: +// 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__) && \ + ABSL_INTERNAL_HAS_WARNING("-Wmissing-variable-declarations") +#define ABSL_INTERNAL_EXTERN_DECL(type, name) \ + extern const ::absl::internal::identity_t<type> name; +#else // Otherwise, just define the macro to do nothing. +#define ABSL_INTERNAL_EXTERN_DECL(type, name) +#endif // defined(__clang__) && + // ABSL_INTERNAL_HAS_WARNING("-Wmissing-variable-declarations") + +// See above comment at top of file for details. +#define ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) \ + ABSL_INTERNAL_EXTERN_DECL(type, name) \ + inline constexpr ::absl::internal::identity_t<type> name = init + +#else // Otherwise, we need to emulate inline variables... + +// See above comment at top of file for details. +// +// Note: +// 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(var_type, name, init) \ + template <class /*AbslInternalDummy*/ = void> \ + struct AbslInternalInlineVariableHolder##name { \ + static constexpr ::absl::internal::identity_t<var_type> kInstance = init; \ + }; \ + \ + template <class AbslInternalDummy> \ + constexpr ::absl::internal::identity_t<var_type> \ + AbslInternalInlineVariableHolder##name<AbslInternalDummy>::kInstance; \ + \ + static constexpr const ::absl::internal::identity_t<var_type>& \ + name = /* NOLINT */ \ + AbslInternalInlineVariableHolder##name<>::kInstance; \ + static_assert(sizeof(void (*)(decltype(name))) != 0, \ + "Silence unused variable warnings.") + +#endif // defined(__cpp_inline_variables) && + // !ABSL_INTERNAL_HAS_WARNING("-Wc++98-c++11-c++14-compat") + +#endif // ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_ |