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SERVER-36900 Add a
unique_function utility.
The `mongo::unique_function` object is a move-only function object which is useful for capturing move-only lambdas (such as those with `std::unique_ptr` objects) into type-erased function objects that resemble `std::function`.
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| /** | ||
| * Copyright 2018 MongoDB, Inc. | ||
| * | ||
| * This program is free software: you can redistribute it and/or modify | ||
| * it under the terms of the GNU Affero General Public License, version 3, | ||
| * as published by the Free Software Foundation. | ||
| * | ||
| * This program is distributed in the hope that it will be useful, | ||
| * but WITHOUT ANY WARRANTY; without even the implied warranty of | ||
| * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | ||
| * GNU Affero General Public License for more details. | ||
| * | ||
| * You should have received a copy of the GNU Affero General Public License | ||
| * along with this program. If not, see <http://www.gnu.org/licenses/>. | ||
| * | ||
| * As a special exception, the copyright holders give permission to link the | ||
| * code of portions of this program with the OpenSSL library under certain | ||
| * conditions as described in each individual source file and distribute | ||
| * linked combinations including the program with the OpenSSL library. You | ||
| * must comply with the GNU Affero General Public License in all respects | ||
| * for all of the code used other than as permitted herein. If you modify | ||
| * file(s) with this exception, you may extend this exception to your | ||
| * version of the file(s), but you are not obligated to do so. If you do not | ||
| * wish to do so, delete this exception statement from your version. If you | ||
| * delete this exception statement from all source files in the program, | ||
| * then also delete it in the license file. | ||
| */ | ||
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| #pragma once | ||
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| #include <functional> | ||
| #include <type_traits> | ||
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| #include "mongo/stdx/type_traits.h" | ||
| #include "mongo/util/assert_util.h" | ||
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| namespace mongo { | ||
| template <typename Function> | ||
| class unique_function; | ||
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| /** | ||
| * A `unique_function` is a move-only, type-erased functor object similar to `std::function`. | ||
| * It is useful in situations where a functor cannot be wrapped in `std::function` objects because | ||
| * it is incapable of being copied. Often this happens with C++14 or later lambdas which capture a | ||
| * `std::unique_ptr` by move. The interface of `unique_function` is nearly identical to | ||
| * `std::function`, except that it is not copyable. | ||
| */ | ||
| template <typename RetType, typename... Args> | ||
| class unique_function<RetType(Args...)> { | ||
| private: | ||
| // `nilbase` is used as a base for the `nil` type, to prevent it from being an aggregate. | ||
| struct nilbase { | ||
| protected: | ||
| nilbase() = default; | ||
| }; | ||
| // `nil` is used as a placeholder type in parameter lists for `enable_if` clauses. They have to | ||
| // be real parameters, not template parameters, due to MSVC limitations. | ||
| class nil : nilbase { | ||
| nil() = default; | ||
| friend unique_function; | ||
| }; | ||
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| public: | ||
| using result_type = RetType; | ||
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| ~unique_function() noexcept = default; | ||
| unique_function() = default; | ||
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| unique_function(const unique_function&) = delete; | ||
| unique_function& operator=(const unique_function&) = delete; | ||
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| unique_function(unique_function&&) noexcept = default; | ||
| unique_function& operator=(unique_function&&) noexcept = default; | ||
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| void swap(unique_function& that) noexcept { | ||
| using std::swap; | ||
| swap(this->impl, that.impl); | ||
| } | ||
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| friend void swap(unique_function& a, unique_function& b) noexcept { | ||
| a.swap(b); | ||
| } | ||
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| // TODO: Look into creating a mechanism based upon a unique_ptr to `void *`-like state, and a | ||
| // `void *` accepting function object. This will permit reusing the core impl object when | ||
| // converting between related function types, such as | ||
| // `int (std::string)` -> `void (const char *)` | ||
| template <typename Functor> | ||
| /* implicit */ | ||
| unique_function( | ||
| Functor&& functor, | ||
| // The remaining arguments here are only for SFINAE purposes to enable this ctor when our | ||
| // requirements are met. They must be concrete parameters not template parameters to work | ||
| // around bugs in some compilers that we presently use. We may be able to revisit this | ||
| // design after toolchain upgrades for C++17. | ||
| std::enable_if_t<stdx::is_invocable_r<RetType, Functor, Args...>::value, nil> = makeNil(), | ||
| std::enable_if_t<std::is_move_constructible<Functor>::value, nil> = makeNil(), | ||
| std::enable_if_t<!std::is_same<Functor, unique_function>::value, nil> = makeNil()) | ||
| : impl(makeImpl(std::forward<Functor>(functor))) {} | ||
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| unique_function(std::nullptr_t) noexcept {} | ||
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| RetType operator()(Args... args) const { | ||
| invariant(static_cast<bool>(*this)); | ||
| return impl->call(std::forward<Args>(args)...); | ||
| } | ||
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| explicit operator bool() const noexcept { | ||
| return static_cast<bool>(this->impl); | ||
| } | ||
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| // Needed to make `std::is_convertible<mongo::unique_function<...>, std::function<...>>` be | ||
| // `std::false_type`. `mongo::unique_function` objects are not convertible to any kind of | ||
| // `std::function` object, since the latter requires a copy constructor, which the former does | ||
| // not provide. If you see a compiler error which references this line, you have tried to | ||
| // assign a `unique_function` object to a `std::function` object which is impossible -- please | ||
| // check your variables and function signatures. | ||
| // | ||
| // NOTE: This is not quite able to disable all `std::function` conversions on MSVC, at this | ||
| // time. | ||
| template <typename Signature> | ||
| operator std::function<Signature>() const = delete; | ||
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| private: | ||
| // The `nil` type cannot be constructed as a default function-parameter in Clang. So we use a | ||
| // static member function that initializes that default parameter. | ||
| static nil makeNil() { | ||
| return {}; | ||
| } | ||
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| struct Impl { | ||
| virtual ~Impl() noexcept = default; | ||
| virtual RetType call(Args&&... args) = 0; | ||
| }; | ||
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| // These overload helpers are needed to squelch problems in the `T ()` -> `void ()` case. | ||
| template <typename Functor> | ||
| static void callRegularVoid(const std::true_type isVoid, Functor& f, Args&&... args) { | ||
| // The result of this call is not cast to void, to help preserve detection of | ||
| // `[[nodiscard]]` violations. | ||
| f(std::forward<Args>(args)...); | ||
| } | ||
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| template <typename Functor> | ||
| static RetType callRegularVoid(const std::false_type isNotVoid, Functor& f, Args&&... args) { | ||
| return f(std::forward<Args>(args)...); | ||
| } | ||
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| template <typename Functor> | ||
| static auto makeImpl(Functor&& functor) { | ||
| struct SpecificImpl : Impl { | ||
| explicit SpecificImpl(Functor&& func) : f(std::move(func)) {} | ||
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| RetType call(Args&&... args) override { | ||
| return callRegularVoid(std::is_void<RetType>(), f, std::forward<Args>(args)...); | ||
| } | ||
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| std::decay_t<Functor> f; | ||
| }; | ||
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| return std::make_unique<SpecificImpl>(std::move(functor)); | ||
| } | ||
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| std::unique_ptr<Impl> impl; | ||
| }; | ||
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| template <typename Signature> | ||
| bool operator==(const unique_function<Signature>& lhs, std::nullptr_t) noexcept { | ||
| return !lhs; | ||
| } | ||
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| template <typename Signature> | ||
| bool operator!=(const unique_function<Signature>& lhs, std::nullptr_t) noexcept { | ||
| return static_cast<bool>(lhs); | ||
| } | ||
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| template <typename Signature> | ||
| bool operator==(std::nullptr_t, const unique_function<Signature>& rhs) noexcept { | ||
| return !rhs; | ||
| } | ||
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| template <typename Signature> | ||
| bool operator!=(std::nullptr_t, const unique_function<Signature>& rhs) noexcept { | ||
| return static_cast<bool>(rhs); | ||
| } | ||
| } // namespace mongo |
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