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But, in general, visitation functions should be as lightweight as possible to reduce contention and increase parallelization. In some cases, moving heavy work outside of visitation may be beneficial:
But, in general, visitation functions should be as lightweight as possible to reduce contention and increase parallelization. In some cases, moving heavy work outside of visitation may be beneficial:但通常而言，访问函数应尽可能轻量以减少争用并提升并行性。在某些情况下，将繁重操作移出访问函数可能更优：
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Access to a different container is allowed, though:
Access to a different container is allowed, though:但允许访问其他容器：
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On the other hand, a visitation function can _not_ access the container itself:
On the other hand, a visitation function can _not_ access the container itself:另一方面，访问函数__无法__访问容器本身：
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The visitation function passed by the user (in this case, a lambda function) is executed internally by Boost.Unordered in a thread-safe manner, so it can access the element without worrying about other threads interfering in the process.
The visitation function passed by the user (in this case, a lambda function) is executed internally by Boost.Unordered in a thread-safe manner, so it can access the element without worrying about other threads interfering in the process.用户传递的访问函数（此处为 lambda 函数）由 Boost.Unordered 在内部以线程安全的方式执行，因此该函数可以安全访问目标元素，无需担心其他线程在此过程中造成干扰。
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In a multithreaded scenario, the iterator `it` may be invalid at point B if some other thread issues an `m.erase(k)` operation between A and B. There are designs that can remedy this by making iterators lock the element they point to, but this approach lends itself to high contention and can easily produce deadlocks in a program. `operator[]` has similar concurrency issues, and is not provided by `boost::concurrent_flat_map`/`boost::concurrent_node_map` either. Instead, element access is done through so-called _visitation functions_:
In a multithreaded scenario, the iterator `it` may be invalid at point B if some other thread issues an `m.erase(k)` operation between A and B. There are designs that can remedy this by making iterators lock the element they point to, but this approach lends itself to high contention and can easily produce deadlocks in a program. `operator[]` has similar concurrency issues, and is not provided by多线程场景中，若其他线程在 A 和 B 之间执行 `m.erase(k)` 操作，迭代器 `it` 可能在 B 点失效。虽然存在通过锁定指向元素来修复此问题的设计方案，但这种方法容易引发高竞争并可能导致程序死锁。 `operator++[]++` 也存在类似并发问题，因此 `boost::concurrent_flat_++_++flat++_++map`/ / `boost::concurrent_node_map` either. Instead, element access is done through so-called _visitation functions_:++_++node++_++map` 也未提供该操作。替代方案是通过__访问函数__操作元素：
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The first thing a new user of Boost.Unordered concurrent containers will notice is that these classes _do not provide iterators_ (which makes them technically not https://en.cppreference.com/w/cpp/named_req/Container[Containers^] in the C++ standard sense). The reason for this is that iterators are inherently thread-unsafe. Consider this hypothetical code:
The first thing a new user of Boost.Unordered concurrent containers will notice is that these classes _do not provide iterators_ (which makes them technically not https://en.cppreference.com/w/cpp/named_req/Container[Containers^] in the C++ standard sense). The reason for this is that iterators are inherently thread-unsafe. Consider this hypothetical code:Boost.Unordered 并发容器的新用户首先会注意到：这些类__不提供迭代器__（它们在技术层面上不符合 C{plus}{plus} 标准中的 https://en.cppreference.com/w/cpp/named_req/Container[容器] 定义）。因为迭代器本质上是线程不安全的。请参考以下假设代码：
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Visitation-based API
Visitation-based基于访问的 API
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In the example above, threads access `m` without synchronization, just as we'd do in a single-threaded scenario. In an ideal setting, if a given workload is distributed among _N_ threads, execution is _N_ times faster than with one thread —this limit is never attained in practice due to synchronization overheads and _contention_ (one thread waiting for another to leave a locked portion of the map), but Boost.Unordered concurrent containers are designed to perform with very little overhead and typically achieve _linear scaling_ (that is, performance is proportional to the number of threads up to the number of logical cores in the CPU).
In the example above, threads access `m` without synchronization, just as we'd do in a single-threaded scenario. In an ideal setting, if a given workload is distributed among _N_ threads, execution is _N_ times faster than with one thread —this limit is never attained in practice due to synchronization overheads and _contention_ (one thread waiting for another to leave a locked portion of the map), but Boost.Unordered concurrent containers are designed to perform with very little overhead and typically achieve _linear scaling_ (that is, performance is proportional to the number of threads up to the number of logical cores in the CPU).在上例中，线程无需同步即可访问 `m` ，这与单线程场景中的操作方式一致。在理想情况下，若将给定工作负载分配给 _N_ 个线程，其执行速度相较单线程提升 _N_ 倍——由于同步开销与__争用__的存在（某线程等待其他线程离开容器的锁定区域），实践中无法达到此理论极限。但 Boost.Unordered 并发容器设计为以极低开销运行，通常可实现__线性扩展__（即性能提升与线程数量成正比，直至达到 CPU 的逻辑核心数）。
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Boost.Unordered provides `boost::concurrent_node_set`, `boost::concurrent_node_map`, `boost::concurrent_flat_set` and `boost::concurrent_flat_map`, hash tables that allow concurrent write/read access from different threads without having to implement any synchronzation mechanism on the user's side.
Boost.Unordered provides提供 `boost::concurrent_node_set`, 、 `boost::concurrent_node_map`, 、 `boost::concurrent_flat_set` and和 `boost::concurrent_flat_map`, hash tables that allow concurrent write/read access from different threads without having to implement any synchronzation mechanism on the user's side. ，这些哈希表允许不同线程进行并发读写访问，无需用户实现任何同步机制。
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:idprefix: concurrent_
:idprefix: concurrent_
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[#concurrent] = Concurrent Containers
[#concurrent] = Concurrent Containers= 并发容器
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One notable operation not provided by `boost::concurrent_flat_map`/`boost::concurrent_node_map` is `operator[]`/`at`, which can be replaced, if in a more convoluted manner, by `xref:reference/concurrent_flat_map.adoc#concurrent_flat_map_try_emplace_or_cvisit[try_emplace_or_visit]`.
One notable operation not provided by `boost::concurrent_flat_map`/ / `boost::concurrent_node_map` is未提供的一个关键操作是 `operator[]`/ / `at`, which can be replaced, if in a more convoluted manner, by ` ，该功能可通过 xref:reference/concurrent_flat_map.adoc#concurrent_flat_map_try_emplace_or_cvisit[`try_emplace_or_visit]`.`] 实现替代（该方式更为复杂）。
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of which there are parallelized versions in C++17 compilers with parallel algorithm support. In general, the interface of concurrent containers is derived from that of their non-concurrent counterparts by a fairly straightforward process of replacing iterators with visitation where applicable. If for regular maps `iterator` and `const_iterator` provide mutable and const access to elements, respectively, here visitation is granted mutable or const access depending on the constness of the member function used (there are also `*cvisit` overloads for explicit const visitation); In the case of `boost::concurrent_flat_set`, visitation is always const.
of which there are parallelized versions in C++17 compilers with parallel algorithm support. In general, the interface of concurrent containers is derived from that of their non-concurrent counterparts by a fairly straightforward process of replacing iterators with visitation where applicable. If for regular maps在支持并行算法的 C{plus}{plus}17 编译器中提供并行化版本。通常，并发容器的接口通过将迭代器替换为访问机制的过程从非并发版本派生。对于常规映射， `iterator` and和 `const_iterator` provide mutable and const access to elements, respectively, here visitation is granted mutable or const access depending on the constness of the member function used (there are also `*cvisit` overloads for explicit const visitation); In the case of `boost::concurrent_flat_set`, visitation is always const.分别提供对元素的非常量和常量访问；此处访问权限取决于所用成员函数的常量性（同时提供 `*cvisit` 重载用于显式常量访问）。对于 `boost::concurrent_flat_set` ，访问操作始终为常量。
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(In the second case `f` is only invoked if there's an equivalent element to `obj` in the table, not if insertion is successful). Container traversal is served by:
(In the second case `f` is only invoked if there's an equivalent element to `obj` in the table, not if insertion is successful). Container traversal is served by:（第二种情况中，仅当表中存在与 `obj` 等价元素时才会调用 `f` ，而非在插入成功时调用）。容器遍历通过以下方式实现：
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are transformed to accept a _visitation function_ that is passed such element:
are transformed to accept a _visitation function_ that is passed such element:被改造为接受传递该元素的__访问函数__：
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In place of iterators, Boost.Unordered concurrent containers use _internal visitation_ facilities as a thread-safe substitute. Classical operations returning an iterator to an element already existing in the container, like for instance:
In place of iterators, Boost.Unordered concurrent containers use _internal visitation_ facilities as a thread-safe substitute. Classical operations returning an iterator to an element already existing in the container, like for instance:为替代迭代器，Boost.Unordered 并发容器使用__内部访问__机制作为线程安全的替代方案。传统操作会返回指向容器中既有元素的迭代器（例如下列）：
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Container traversal.
Container traversal.容器遍历。
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Access to an element previously located via lookup.
Access to an element previously located via lookup.访问先前通过查找定位的元素。
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In a non-concurrent unordered container, iterators serve two main purposes:
In a non-concurrent unordered container, iterators serve two main purposes:在非并发无序容器中，迭代器主要有两项核心功能：
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