c++ - std::future::wait 是内存屏障吗？ (我无法解释这个数据竞赛)

Question

代码如下:

std::vector<bool> a(req_count_);
std::vector<std::future<void>> waits(req_count_);

for (int i = 0; i < req_count_; i++) {
  // send into a threadpool implementation
  waits[i] = framework::Async([i, &a] {
    a[i] = true; // write true
  });
}

for (int i = 0; i < req_count_; i++) {
  waits[i].wait(); // memory barrier?
}

int last_req_count = req_count_;
req_count_ = 0;

for (int i = 0; i < last_req_count; i++) {
  if (!a[i]) { // read false
    return false;
  }
}

我的问题是 std::future::wait 是否用作内存屏障？ std::future::wait 等待函数调用完成，但函数是否发生在 std::future::wait (例如，由其他线程可见的函数调用引起的状态突变)？

如果 std::future::wait 不作为内存屏障，我们如何实现线程池以便在 future 完成时自动触发内存屏障？

如果您认为我对内存屏障的理解有误，请指正。

Answer 1

[container.requirements.dataraces]/2 Notwithstanding [res.on.data.races], implementations are required to avoid data races when the contents of the contained object in different elements in the same container, excepting vector<bool>, are modified concurrently.

[container.requirements.dataraces]/3 [ Note: For a vector<int> x with a size greater than one, x[1] = 5 and *x.begin() = 10 can be executed concurrently without a data race, but x[0] = 5 and *x.begin() = 10 executed concurrently may result in a data race. As an exception to the general rule, for a vector<bool> y, y[0] = true may race with y[1] = true. —end note ]

强调我的。比赛发生在 a[i] = true; . vector<bool>不是真正的容器，访问“元素”需要接触相邻元素的位操作。