它是否安全,例如 std :: promise< T> 变为可变,还是取决于 T ?如:
Is it safe, like in the case of std::mutex for a std::promise<T> to be made mutable, or does it depend on T? As in:
using Data = std::tuple<bool, int, int>; struct X { std::future<Data> prepare() const { return m_promise.get_future(); } void asyncHandler(int a, int b) const { m_promise.set_value({true, a, b}); } void cancel() const { m_promise.set_value({false, 0, 0}); } mutable std::promise<Data> m_promise; // Is this safe? }; void performAsyncOp(const X& x) { std::future<Data> fut = x.prepare(); dispatch(x); std::future_status result = fut.wait_for(std::chrono::milliseconds(150)); if (result == std::future_status::timeout) { x.cancel(); } handleResult(fut.get()); }推荐答案
让我们详细看一下API:
Let's have a detailed look at the API:
// retrieving the result future<R> get_future(); // setting the result void set_value(see below); void set_exception(exception_ptr p); // setting the result with deferred notification void set_value_at_thread_exit(see below); void set_exception_at_thread_exit(exception_ptr p);所有方法均未标记为 const ,因此我们不能仅凭此推断出任何关于常数的知识。但是,该标准规定了以下方法的线程安全性(cf 33.6.6.2 ): set_value , set_exception , set_value_at_thread_exit 和 set_exception_at_thread_exit 。
None of the methods is marked const, so we can't infer any knowledge about the constness from just this. However, the standard mandates thread-safety on the following methods (c.f. 33.6.6.2): set_value, set_exception, set_value_at_thread_exit, and set_exception_at_thread_exit.
这使 get_future 的线程未指定-安全。但是,如果c.f多次调用 get_future ,则会引发异常。 33.6.6.14.1 。因此,从实际的角度来看,从多个线程调用 get_future 并没有什么意义。
This leaves get_future unspecified with respect to thread-safety. However, get_future throws an exception if called more than once, c.f. 33.6.6.14.1. So calling get_future from multiple threads doesn't really make sense from a practical point of view.
没有调用 get_future 以及任何 set 方法和 get_future 时的线程安全性保证据我所知,同时code>(无论它是否会抛出)。
There is no guarantee for thread-safety when calling get_future and any of the set methods and get_future (no matter if it will throw or not) simultaneously, as far as I can see.
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是std :: promise< T>线程安全的?
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