c++ - 如何手动创建将在析构函数中阻塞的 future

Question

我读过 http://scottmeyers.blogspot.com/2013/03/stdfutures-from-stdasync-arent-special.html但它没有显示如何实现相同类型行为的示例 - 即 future 将在析构函数中阻塞，与异步返回的析构函数相同，但不使用任何异步。

你能举个例子吗？谢谢!

Answer 1

如果您关注该文章下方的评论线程，您会发现其他专家并不同意:

• 不是来自 async 的 future 应该在析构函数中阻塞。

• 根据 Scott Meyers 对标准的解释，应该是这样。

马蒂尼奥·费尔南德斯:

I don't see why this is a requirement for all futures: this particular requirement you quote is from the requirements upon std::async, not the requirements on std::future. The only requirement on the destructor of std::future is that it "releases the shared state". It happens that the shared state from std::async adds the requirement you quote, but no other shared state in the standard library has such a requirement.

赫伯萨特:

tl;dr: Martinjo already answered this correctly -- the article is not correct, the blocking applies only to futures returned from std::async with launch policy launch::async.

Scott said: and in 30.6.8/5, we see that [...] This is a requirement for any future object, not just the ones returned from std::async calls.

这不成立。 30.6.8 是 std::async 的规范。 其中所说的任何内容都是针对 std::async 的。

但正如 Martinho 已经正确指出的那样，此异常仅出现在第 30.6.8 条“函数模板异步”中，一般不适用于 future 。

结论:我认为您无法找到您正在寻找的示例。