c++ - 如何理解 "std::future shared state"？

Question

根据文档(http://www.cplusplus.com/reference/future/future/get/)，它说: [强调我的]

Once the shared state is ready, the function unblocks and returns (or throws) releasing its shared state. This makes the future object no longer valid: this member function shall be called once at most for every future shared state.

根据文档(http://www.cplusplus.com/reference/future/future/)，它说: [强调我的]

"Valid" futures are future objects associated to a shared state, and are constructed by calling one of the following functions

async

promise::get_future

packaged_task::get_future

根据文档(http://www.cplusplus.com/reference/future/future/get/)，它说: [强调我的]

std::future::get

generic template (1) T get(); reference

specialization (2) R& future::get(); // when T is a reference

type (R&) void specialization (3) void future::get(); // when T is void

返回:

value Generally (1), std::move(x), where x is the value stored in the shared state.

For references (2), a reference to the value stored in the shared state.

For void futures (3), nothing.

我的问题是什么是“std::future 共享状态”？我如何理解“std::future 共享状态”？

我是C++的新手，想了又想，还是领悟不到思路。如果能为这个问题提供一些帮助，我将不胜感激。

Answer 1

根据cppreference std::future 的文档:

When the asynchronous operation is ready to send a result to the creator, it can do so by modifying shared state (e.g. std::promise::set_value) that is linked to the creator's std::future.

这里的共享状态指的是一些多线程可读/可写的可变内容。考虑直接来自文档的示例:

std::promise<int> p;
std::future<int> f = p.get_future();
std::thread([&p]{ p.set_value_at_thread_exit(9); }).detach();
f.wait();

主线程和创建的线程共享p(因为p是有状态的，它们共享状态!)。在 lambda 主体内调用 std::promise::set_value_at_thread_exit 有效地修改了这两个线程之间的共享状态。这基本上就是所有的意思。

promise 给出的 std::future 表示共享状态的“有趣”部分——您关心的值。