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< a class = "header" href = "#使用sync和send-trait-的可扩展并发" name = "使用sync和send-trait-的可扩展并发" > < h2 > 使用< code > Sync< / code > 和< code > Send< / code > trait 的可扩展并发< / h2 > < / a >
< blockquote >
< p > < a href = "https://github.com/rust-lang/book/blob/master/second-edition/src/ch16-04-extensible-concurrency-sync-and-send.md" > ch16-04-extensible-concurrency-sync-and-send.md< / a >
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commit 9430a3d28a2121a938d704ce48b15d21062f880e< / p >
< / blockquote >
< p > Rust 的并发模型中一个有趣的方面是：语言本身对并发知之< strong > 甚少< / strong > 。我们之前讨论的几乎所有内容，都属于标准库，而不是语言本身的内容。由于不需要语言提供并发相关的基础设施，并发方案不受标准库或语言所限：我们可以编写自己的或使用别人编写的。< / p >
< p > 我们说“< strong > 几乎< / strong > 所有内容都不属于语言本身”，那么属于语言本身的是什么呢？是两个 trait， < code > std::marker< / code > ： < code > Sync< / code > 和< code > Send< / code > 。< / p >
< a class = "header" href = "#send用于表明所有权可能被传送给其他线程" name = "send用于表明所有权可能被传送给其他线程" > < h3 > < code > Send< / code > 用于表明所有权可能被传送给其他线程< / h3 > < / a >
< p > < code > Send< / code > 标记 trait 表明类型的所有权可能被在线程间传递。几乎所有的 Rust 类型都是< code > Send< / code > 的，不过有一些例外。比如标准库中提供的 < code > Rc< T> < / code > ：如果克隆< code > Rc< T> < / code > 值，并尝试将克隆的所有权传递给另一个线程，这两个线程可能会同时更新引用计数。正如上一部分提到的，< code > Rc< T> < / code > 被实现为用于单线程场景，这时不需要为拥有线程安全的引用计数而付出性能代价。< / p >
< p > 因为< code > Rc< T> < / code > 没有标记为< code > Send< / code > ， < code > Rc< T> < / code > 值不安全的在线程间传递。列表 16-14 曾尝试这么做，不过得到了一个错误，< code > the trait Send is not implemented for Rc< Mutex< i32> > < / code > 。当切换为标记为< code > Send< / code > 的< code > Arc< T> < / code > 时，就没有问题了。< / p >
< p > 任何完全由< code > Send< / code > 的类型组成的类型也会自动被标记为< code > Send< / code > 。几乎所有基本类型都是< code > Send< / code > 的，大部分标准库类型是< code > Send< / code > 的，除了< code > Rc< T> < / code > ， （ ） < / p >
< a class = "header" href = "#sync表明多线程访问是安全的" name = "sync表明多线程访问是安全的" > < h3 > < code > Sync< / code > 表明多线程访问是安全的< / h3 > < / a >
< p > < code > Sync< / code > 标记 trait 表明一个类型可以安全的在多个线程中拥有其值的引用。换一种方式来说就是，对于任意类型< code > T< / code > ，如果< code > & T< / code > （ < code > T< / code > 的引用）是< code > Send< / code > 的话< code > T< / code > 就是< code > Sync< / code > 的，这样其引用就可以安全的发送到另一个线程。类似于< code > Send< / code > 的情况，基本类型是< code > Sync< / code > 的，完全由< code > Sync< / code > 的类型组成的类型也是< code > Sync< / code > 的。< / p >
< p > < code > Rc< T> < / code > 也不是< code > Sync< / code > 的，出于其不是< code > Send< / code > 的相同的原因。< code > RefCell< T> < / code > （第十五章讨论过）和< code > Cell< T> < / code > 系列类型不是< code > Sync< / code > 的。< code > RefCell< T> < / code > 在运行时所进行的借用检查也不是线程安全的。< code > Mutex< T> < / code > 是< code > Sync< / code > 的，正如上一部分所讲的它可以被用来在多线程中共享访问。< / p >
< a class = "header" href = "#手动实现send和sync是不安全的" name = "手动实现send和sync是不安全的" > < h3 > 手动实现< code > Send< / code > 和< code > Sync< / code > 是不安全的< / h3 > < / a >
< p > 通常并不需要实现< code > Send< / code > 和< code > Sync< / code > trait， < code > Send< / code > 和< code > Sync< / code > 的类型组成的类型，自动就是< code > Send< / code > 和< code > Sync< / code > 的。因为他们是标记 trait， < / p >
< p > 实现这些标记 trait 涉及到编写不安全的 Rust 代码，第十九章将会讲述具体的方法；当前重要的是，在创建新的由不是< code > Send< / code > 和< code > Sync< / code > 的部分构成的并发类型时需要多加小心，以确保维持其安全保证。< a href = "https://doc.rust-lang.org/stable/nomicon/" > The Nomicon< / a > 中有更多关于这些保证以及如何维持他们的信息。< / p >
< a class = "header" href = "#总结" name = "总结" > < h2 > 总结< / h2 > < / a >
< p > 这不会是本书最后一个出现并发的章节；第二十章的项目会在更现实的场景中使用这些概念，而不像本章中讨论的这些小例子。< / p >
< p > 正如我们提到的，因为 Rust 本身很少有处理并发的部分内容，有很多的并发方案都由 crate 实现。他们比标准库要发展的更快；请在网上搜索当前最新的用于多线程场景的 crate。< / p >
< p > Rust 提供了用于消息传递的通道，和像< code > Mutex< T> < / code > 和< code > Arc< T> < / code > 这样可以安全的用于并发上下文的智能指针。类型系统和借用检查器会确保这些场景中的代码，不会出现数据竞争和无效的引用。一旦代码可以编译了，我们就可以坚信这些代码可以正确的运行于多线程环境，而不会出现其他语言中经常出现的那些难以追踪的 bug。并发编程不再是什么可怕的概念： ！ < / p >
< p > 接下来，让我们讨论一下当 Rust 程序变得更大时，有哪些符合语言习惯的问题建模方法和结构化解决方案，以及 Rust 的风格是如何与面向对象编程（ ） < / p >
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