108 lines
4.6 KiB
C++
108 lines
4.6 KiB
C++
// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
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#define ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
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#include <type_traits>
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#include "absl/base/internal/identity.h"
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// File:
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// This file define a macro that allows the creation of or emulation of C++17
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// inline variables based on whether or not the feature is supported.
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////////////////////////////////////////////////////////////////////////////////
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// Macro: ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init)
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//
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// Description:
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// Expands to the equivalent of an inline constexpr instance of the specified
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// `type` and `name`, initialized to the value `init`. If the compiler being
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// used is detected as supporting actual inline variables as a language
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// feature, then the macro expands to an actual inline variable definition.
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//
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// Requires:
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// `type` is a type that is usable in an extern variable declaration.
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//
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// Requires: `name` is a valid identifier
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//
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// Requires:
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// `init` is an expression that can be used in the following definition:
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// constexpr type name = init;
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//
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// Usage:
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//
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// // Equivalent to: `inline constexpr size_t variant_npos = -1;`
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// ABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, -1);
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//
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// Differences in implementation:
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// For a direct, language-level inline variable, decltype(name) will be the
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// type that was specified along with const qualification, whereas for
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// emulated inline variables, decltype(name) may be different (in practice
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// it will likely be a reference type).
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////////////////////////////////////////////////////////////////////////////////
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#ifdef __cpp_inline_variables
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// Clang's -Wmissing-variable-declarations option erroneously warned that
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// inline constexpr objects need to be pre-declared. This has now been fixed,
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// but we will need to support this workaround for people building with older
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// versions of clang.
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//
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// Bug: https://bugs.llvm.org/show_bug.cgi?id=35862
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//
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// Note:
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// identity_t is used here so that the const and name are in the
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// appropriate place for pointer types, reference types, function pointer
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// types, etc..
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#if defined(__clang__)
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#define ABSL_INTERNAL_EXTERN_DECL(type, name) \
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extern const ::absl::internal::identity_t<type> name;
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#else // Otherwise, just define the macro to do nothing.
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#define ABSL_INTERNAL_EXTERN_DECL(type, name)
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#endif // defined(__clang__)
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// See above comment at top of file for details.
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#define ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) \
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ABSL_INTERNAL_EXTERN_DECL(type, name) \
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inline constexpr ::absl::internal::identity_t<type> name = init
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#else
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// See above comment at top of file for details.
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//
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// Note:
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// identity_t is used here so that the const and name are in the
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// appropriate place for pointer types, reference types, function pointer
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// types, etc..
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#define ABSL_INTERNAL_INLINE_CONSTEXPR(var_type, name, init) \
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template <class /*AbslInternalDummy*/ = void> \
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struct AbslInternalInlineVariableHolder##name { \
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static constexpr ::absl::internal::identity_t<var_type> kInstance = init; \
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}; \
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\
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template <class AbslInternalDummy> \
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constexpr ::absl::internal::identity_t<var_type> \
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AbslInternalInlineVariableHolder##name<AbslInternalDummy>::kInstance; \
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\
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static constexpr const ::absl::internal::identity_t<var_type>& \
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name = /* NOLINT */ \
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AbslInternalInlineVariableHolder##name<>::kInstance; \
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static_assert(sizeof(void (*)(decltype(name))) != 0, \
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"Silence unused variable warnings.")
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#endif // __cpp_inline_variables
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#endif // ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
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