65 lines
2.5 KiB
C++
65 lines
2.5 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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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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* http://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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*/
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#pragma once
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#include <type_traits>
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// FOLLY_CREATE_EXTERN_ACCESSOR
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//
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// Defines a variable template which holds the address of an object or null
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// given a condition which identifies whether that object is expected to have
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// a definition.
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//
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// Example:
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//
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// extern void foobar(int); // may be defined in another library
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// inline constexpr bool has_foobar = // ...
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//
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// namespace myapp {
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// FOLLY_CREATE_EXTERN_ACCESSOR(access_foobar_v, ::foobar);
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// void try_foobar(int num) {
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// if (auto ptr = access_foobar_v<has_foobar>) {
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// ptr(num);
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// }
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// }
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// }
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//
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// Remarks:
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//
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// This can be done more simply using weak symbols. But weak symbols are non-
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// portable and the rules around the use of weak symbols are complex.
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//
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// This can be done more simply using if-constexpr. But if-constexpr requires
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// C++17 and there remain specific narrow cases to be supported on prior
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// versions of C++.
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//
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// TODO: Remove after C++17.
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#define FOLLY_CREATE_EXTERN_ACCESSOR(varname, name) \
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struct __folly_extern_accessor_##varname { \
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template <bool E, typename N, ::std::enable_if_t<E, int> = 0> \
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static constexpr N* get() noexcept { \
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return &name; \
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} \
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template <bool E, typename N, ::std::enable_if_t<!E, int> = 0> \
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static constexpr N* get() noexcept { \
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return nullptr; \
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} \
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}; \
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template <bool E, typename N = decltype(name)> \
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FOLLY_INLINE_VARIABLE constexpr N* varname = \
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__folly_extern_accessor_##varname::get<E, N>()
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