mirror of
https://github.com/facebook/react-native.git
synced 2025-11-01 09:14:26 +00:00
Fabrizio Cucci
ad52137183
Summary: Pull Request resolved: https://github.com/facebook/react-native/pull/39751 There doesn't seem to be an easy way to drop `folly::makeMoveWrapper` for the reasons explained in below in the diff. To unblock the folly deprecation in the RN codebase, we are copying the `MoveWrapper` class within the RN codebase. Changelog: [Internal] Reviewed By: sammy-SC, cipolleschi Differential Revision: D49423389 fbshipit-source-id: 8abaa95a6d675b069be8e74933aa8c63f4ea43ee
126 lines
4.3 KiB
C++
126 lines
4.3 KiB
C++
/*
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* Copyright (c) Meta Platforms, Inc. and affiliates.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*/
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#pragma once
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#include <memory>
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namespace facebook::react {
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/*
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NOTE: we keep this internal copy of folly/MoveWrapper.h to unblock
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the the workstream of dropping the dependency on folly in RN!
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For a technical explanation on why we still need this we defer
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to the doc in folly/Function.h:
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"There are some limitations in std::function that folly::Function tries to
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avoid. std::function is copy-constructible and requires that the callable that
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it wraps is copy-constructible as well, which is a constraint that is often
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inconvenient. In most cases when using a std::function you don't make use of
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its copy-constructibility, so you might sometimes feel like you get back very
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little in return for a noticeable restriction. This restriction becomes
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apparent when trying to use a lambda capturing a unique_ptr (or any
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non-copyable type) as a callback for a folly::Future.
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std::unique_ptr<Foo> foo_ptr = new Foo;
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some_future.then(
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[foo_ptr = std::move(foo_ptr)] mutable
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(int x)
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{ foo_ptr->setX(x); }
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);
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This piece of code did not compile before folly::Future started using
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folly::Function instead of std::function to store the callback. Because the
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lambda captures something non-copyable (the unique_ptr), it is not copyable
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itself. And std::function can only store copyable callables.
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The implementation of folly::Future did not make use of the
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copy-constructibility of std::function at any point. There was no benefit from
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the fact that the std::function is copy-constructible, but the fact that it can
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only wrap copy-constructible callables posed a restriction.
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A workaround was available: folly::MoveWrapper, which wraps an object that may
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be non-copyable and implements copy operations by moving the embedded object.
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Using a folly::MoveWrapper, you can capture non-copyable objects in a lambda,
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and the lambda itself is still copyable and may be wrapped in a std::function.
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It is a pragmatic solution for the above problem, but you have to be a little
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careful. The problem is that you can’t use a MoveWrapper anywhere where copy
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operations are assumed to behave like actual copy operations. Also, a
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folly::MoveWrapper<std::unique_ptr<T>> essentially behaves like auto_ptr<T>. Ask
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yourself whether you’d want to use lots of auto_ptrs in your codebase. And the
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original question still persists: we very often don’t benefit from
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copy-constructibility of std::function, so why do we have to live with this
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restriction? I.e. why do we have to use MoveWrapper?"
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*/
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/** C++11 closures don't support move-in capture. Nor does std::bind.
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facepalm.
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http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3610.html
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"[...] a work-around that should make people's stomach crawl:
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write a wrapper that performs move-on-copy, much like the deprecated
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auto_ptr"
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Unlike auto_ptr, this doesn't require a heap allocation.
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*/
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template <class T>
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class MoveWrapper {
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public:
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/** If value can be default-constructed, why not?
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Then we don't have to move it in */
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MoveWrapper() = default;
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/// Move a value in.
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explicit MoveWrapper(T&& t) : value(std::move(t)) {}
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/// copy is move
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MoveWrapper(const MoveWrapper& other) : value(std::move(other.value)) {}
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/// move is also move
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MoveWrapper(MoveWrapper&& other) noexcept : value(std::move(other.value)) {}
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const T& operator*() const {
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return value;
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}
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T& operator*() {
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return value;
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}
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const T* operator->() const {
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return &value;
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}
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T* operator->() {
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return &value;
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}
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/// move the value out (sugar for std::move(*moveWrapper))
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T&& move() {
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return std::move(value);
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}
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// If you want these you're probably doing it wrong, though they'd be
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// easy enough to implement
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MoveWrapper& operator=(MoveWrapper const&) = delete;
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MoveWrapper& operator=(MoveWrapper&&) = delete;
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private:
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mutable T value;
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};
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/// Make a MoveWrapper from the argument. Because the name "makeMoveWrapper"
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/// is already quite transparent in its intent, this will work for lvalues as
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/// if you had wrapped them in std::move.
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template <class T, class T0 = typename std::remove_reference<T>::type>
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MoveWrapper<T0> makeMoveWrapper(T&& t) {
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return MoveWrapper<T0>(std::forward<T0>(t));
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}
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} // namespace facebook::react
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