因为推文感觉问题描述不清楚，乾脆回一篇问问题 看完这三个policy之後对於他的设计感到有点疑惑，因此跑回去看draft 这是今年二月的版本 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0024r1.html 前一篇已经把这个草稿的精髓描述过了 我在这边只节录草稿中对於每个policy效果的描述 ##sequential_execution_policy The invocations of element access functions in parallel algorithms invoked with an execution policy object of type sequential_execution_policy are indeterminately sequenced (in) the calling thread. 基本上似乎是个fallback用的policy，让你在不改变呼叫形式的情况下也可以避免平行化 只是有个疑问，为什麽要设计成「不保证执行顺序 (indeterminately sequenced)」呢？ 有趣的是在2015的初稿中，原本是保证依序执行的，不知甚麽原因被改掉了 ##parallel_execution_policy The invocations of element access functions in parallel algorithms invoked with an execution policy object of type parallel_execution_policy are permitted to execute in an unordered fashion in either the invoking thread or in a thread implicitly created by the library to support parallel algorithm execution. 还蛮好想像的行为，没甚麽问题 ##parallel_vector_execution_policy The invocations of element access functions in parallel algorithms invoked with an execution policy of type parallel_vector_execution_policy are permitted to execute in an unordered fashion in unspecified threads, and unsequenced with respect to one another within each thread. 问题来了，这个policy的优点何在？ 乍看之下与parallel_execution_policy相较没有额外的好处，还会引入很多问题 例如前篇也提过的，许多synchronization都可能会变成deadlock 甚麽样的情况下会不用上一个policy而采用这个？ --

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