77 lines
2.5 KiB
C++
77 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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#include <folly/Portability.h>
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#include <folly/lang/StaticConst.h>
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// FOLLY_DEFINE_CPO
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//
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// Helper for portably defining customization-point objects (CPOs).
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//
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// The customization-point object must be placed in a nested namespace to avoid
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// potential conflicts with customizations defined as friend-functions of types
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// defined in the same namespace as the CPO.
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//
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// In C++17 and later the object may be defined using 'inline constexpr' to
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// avoid ODR issues. However, prior to that a helper template is required to
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// ensure that there is only a single instance of the CPO created and then a
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// named reference in an anonymous namespace is required to avoid duplicate
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// symbol definitions.
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#if FOLLY_HAS_INLINE_VARIABLES
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#define FOLLY_DEFINE_CPO(Type, Name) \
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namespace folly_cpo__ { \
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inline constexpr Type Name{}; \
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} \
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using namespace folly_cpo__;
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#else
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#define FOLLY_DEFINE_CPO(Type, Name) \
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namespace { \
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constexpr auto& Name = ::folly::StaticConst<Type>::value; \
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}
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#endif
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namespace folly {
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// Using 'auto' for non-type template parameters is only possible from C++17
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#if FOLLY_CPLUSPLUS >= 201703L
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// cpo_t<CPO>
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//
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// Helper type-trait for obtaining the type of customisation point object.
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//
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// This can be useful for defining overloads of tag_invoke() where CPOs
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// are defined in terms of ADL calls to tag_invoke().
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//
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// For example:
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// FOLLY_DEFINE_CPO(DoSomething_fn, doSomething)
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//
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// class SomeClass {
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// ...
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// friend void tag_invoke(folly::cpo_t<doSomething>, const SomeClass& x);
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// };
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//
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// See <folly/functional/Invoke.h> for more details.
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template <const auto& Tag>
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using cpo_t = std::decay_t<decltype(Tag)>;
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#endif // FOLLY_CPLUSPLUS >= 201703L
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} // namespace folly
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