107 lines
4.6 KiB
C++
107 lines
4.6 KiB
C++
// 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_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).
|
|
////////////////////////////////////////////////////////////////////////////////
|
|
|
|
#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:
|
|
// 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) \
|
|
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__)
|
|
|
|
// 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
|
|
|
|
// 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 // __cpp_inline_variables
|
|
|
|
#endif // ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
|