inline_variable.h
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00001 // Copyright 2017 The Abseil Authors.
00002 //
00003 // Licensed under the Apache License, Version 2.0 (the "License");
00004 // you may not use this file except in compliance with the License.
00005 // You may obtain a copy of the License at
00006 //
00007 //      https://www.apache.org/licenses/LICENSE-2.0
00008 //
00009 // Unless required by applicable law or agreed to in writing, software
00010 // distributed under the License is distributed on an "AS IS" BASIS,
00011 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
00012 // See the License for the specific language governing permissions and
00013 // limitations under the License.
00014 
00015 #ifndef ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
00016 #define ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
00017 
00018 #include <type_traits>
00019 
00020 #include "absl/base/internal/identity.h"
00021 
00022 // File:
00023 //   This file define a macro that allows the creation of or emulation of C++17
00024 //   inline variables based on whether or not the feature is supported.
00025 
00027 // Macro: ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init)
00028 //
00029 // Description:
00030 //   Expands to the equivalent of an inline constexpr instance of the specified
00031 //   `type` and `name`, initialized to the value `init`. If the compiler being
00032 //   used is detected as supporting actual inline variables as a language
00033 //   feature, then the macro expands to an actual inline variable definition.
00034 //
00035 // Requires:
00036 //   `type` is a type that is usable in an extern variable declaration.
00037 //
00038 // Requires: `name` is a valid identifier
00039 //
00040 // Requires:
00041 //   `init` is an expression that can be used in the following definition:
00042 //     constexpr type name = init;
00043 //
00044 // Usage:
00045 //
00046 //   // Equivalent to: `inline constexpr size_t variant_npos = -1;`
00047 //   ABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, -1);
00048 //
00049 // Differences in implementation:
00050 //   For a direct, language-level inline variable, decltype(name) will be the
00051 //   type that was specified along with const qualification, whereas for
00052 //   emulated inline variables, decltype(name) may be different (in practice
00053 //   it will likely be a reference type).
00055 
00056 #ifdef __cpp_inline_variables
00057 
00058 // Clang's -Wmissing-variable-declarations option erroneously warned that
00059 // inline constexpr objects need to be pre-declared. This has now been fixed,
00060 // but we will need to support this workaround for people building with older
00061 // versions of clang.
00062 //
00063 // Bug: https://bugs.llvm.org/show_bug.cgi?id=35862
00064 //
00065 // Note:
00066 //   identity_t is used here so that the const and name are in the
00067 //   appropriate place for pointer types, reference types, function pointer
00068 //   types, etc..
00069 #if defined(__clang__)
00070 #define ABSL_INTERNAL_EXTERN_DECL(type, name) \
00071   extern const ::absl::internal::identity_t<type> name;
00072 #else  // Otherwise, just define the macro to do nothing.
00073 #define ABSL_INTERNAL_EXTERN_DECL(type, name)
00074 #endif  // defined(__clang__)
00075 
00076 // See above comment at top of file for details.
00077 #define ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) \
00078   ABSL_INTERNAL_EXTERN_DECL(type, name)                  \
00079   inline constexpr ::absl::internal::identity_t<type> name = init
00080 
00081 #else
00082 
00083 // See above comment at top of file for details.
00084 //
00085 // Note:
00086 //   identity_t is used here so that the const and name are in the
00087 //   appropriate place for pointer types, reference types, function pointer
00088 //   types, etc..
00089 #define ABSL_INTERNAL_INLINE_CONSTEXPR(var_type, name, init)                  \
00090   template <class /*AbslInternalDummy*/ = void>                               \
00091   struct AbslInternalInlineVariableHolder##name {                             \
00092     static constexpr ::absl::internal::identity_t<var_type> kInstance = init; \
00093   };                                                                          \
00094                                                                               \
00095   template <class AbslInternalDummy>                                          \
00096   constexpr ::absl::internal::identity_t<var_type>                            \
00097       AbslInternalInlineVariableHolder##name<AbslInternalDummy>::kInstance;   \
00098                                                                               \
00099   static constexpr const ::absl::internal::identity_t<var_type>&              \
00100       name = /* NOLINT */                                                     \
00101       AbslInternalInlineVariableHolder##name<>::kInstance;                    \
00102   static_assert(sizeof(void (*)(decltype(name))) != 0,                        \
00103                 "Silence unused variable warnings.")
00104 
00105 #endif  // __cpp_inline_variables
00106 
00107 #endif  // ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_


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autogenerated on Wed Jun 19 2019 19:42:14