April 2018
Intermediate to advanced
303 pages
4h 54m
Chinese
Content preview from Effective Modern C++（中文版）
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并发
API
｜
243
条款
38
：对变化多端的线程句柄析构函数行为保
持关注
条款
37
解释过，可联结的线程对应着一个底层系统执行线程，未推迟任务（参见条
款
36
）的期值和系统线程也有类似关系。这么一来，
std::thread
型别对象和期值对
象都可以视作系统线程的句柄。
从这个视角来看，
std::thread
对象和期值对象的析构函数表现出如此不同的行为值
得深思。正如条款
37
所提及的，针对可联结的
std::thread
型别对象实施析构会导
致程序终止，因为另外两个显而易见的选择（隐式
join
和隐式
detach
）都被认为是
更糟糕的选择。而期值的析构函数呢，有时候行为像是执行了一次隐式
join
，有时候
行为像是执行了一次隐式
detach
，有时候行为像是二者都没有执行。它从不会导致程
序终止。这套线程句柄行为的大杂烩，值得我们仔细品鉴一番。
我们的观察从这里开始：期值位于信道的一端，被调方把结果通过该信道传输给调用
方。
注
4
被调方（通常以异步方式运行）把其计算所得的结果写入信道（通常经由一
个
std::promise
型别对象），而调用方则使用一个期值来读取该结果。你可以把这
个过程想像成下图，虚线箭头代表着从被调方向调用方的信息流：
调用方 被调方
期值
（一般情况）
但被调方的结果要存储在哪里呢？在调用方唤起对应期值的
get
之前，被调方可能已
经执行完毕，因此结果不会存储在被调方的
std::promise
型别对象里。那个对象，对
于被调方来说是个局部量，在被调方结束后会实施析构。
该结果也不能存储在调用方的期值中，因为
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.