75 lines
2.3 KiB
C++
75 lines
2.3 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 <utility>
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#include <folly/functional/Invoke.h>
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#include <folly/synchronization/DelayedInit.h>
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namespace folly {
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/*
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* ConcurrentLazy is for thread-safe, delayed initialization of a value. This
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* combines the benefits of both `folly::Lazy` and `folly::DelayedInit` to
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* compute the value, once, at access time.
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*
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* There are a few differences between the non-concurrent Lazy, most notably:
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*
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* - this only safely initializes the value; thread-safety of the underlying
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* value is left up to the caller.
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* - the underlying types are not copyable or moveable, which means that this
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* type is also not copyable or moveable.
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*
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* Otherwise, all design considerations from `folly::Lazy` are reflected here.
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*/
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template <class Ctor>
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struct ConcurrentLazy {
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using result_type = invoke_result_t<Ctor>;
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static_assert(
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!std::is_const<Ctor>::value, "Func should not be a const-qualified type");
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static_assert(
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!std::is_reference<Ctor>::value, "Func should not be a reference type");
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template <
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typename F,
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std::enable_if_t<std::is_constructible_v<Ctor, F>, int> = 0>
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explicit ConcurrentLazy(F&& f) noexcept(
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std::is_nothrow_constructible_v<Ctor, F>)
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: ctor_(static_cast<F&&>(f)) {}
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const result_type& operator()() const {
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return value_.try_emplace_with(std::ref(ctor_));
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}
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result_type& operator()() { return value_.try_emplace_with(std::ref(ctor_)); }
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private:
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mutable folly::DelayedInit<result_type> value_;
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mutable Ctor ctor_;
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};
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template <class Func>
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ConcurrentLazy<remove_cvref_t<Func>> concurrent_lazy(Func&& func) {
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return ConcurrentLazy<remove_cvref_t<Func>>(static_cast<Func&&>(func));
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}
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} // namespace folly
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