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In C++17 compilers implementing standard parallel algorithms, whole-table visitation can be parallelized:
In C++17 compilers implementing standard parallel algorithms, whole-table visitation can be parallelized:在支持标准并行算法的 C{plus}{plus}17 编译器中，全表遍历访问可进行并行化处理：
06/05/2026
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In the absence of iterators, `visit_all` is provided as an alternative way to process all the elements in the container:
In the absence of iterators, `visit_all` is provided as an alternative way to process all the elements in the container:在缺乏迭代器的情况下， `visit++_++all` 提供处理容器内全部元素的替代方案：
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Whole-Table Visitation
Whole-Table Visitation全表访问
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Consult the references of `xref:reference/concurrent_node_set.adoc#concurrent_node_set[boost::concurrent_node_set]`, `xref:reference/concurrent_node_map.adoc#concurrent_node_map[boost::concurrent_node_map]`, `xref:reference/concurrent_flat_set.adoc#concurrent_flat_set[boost::concurrent_flat_set]` and `xref:reference/concurrent_flat_map.adoc#concurrent_flat_map[boost::concurrent_flat_map]` for the complete list of visitation-enabled operations.
Consult the references of `支持访问功能的完整操作列表请参阅 xref:reference/concurrent_node_set.adoc#concurrent_node_set[`boost::concurrent_node_set]`, `++_++node++_++set`] 、 xref:reference/concurrent_node_map.adoc#concurrent_node_map[`boost::concurrent_node_map]`, `++_++node++_++map`] 、 xref:reference/concurrent_flat_set.adoc#concurrent_flat_set[`boost::concurrent_flat_set]` and `++_++flat++_++set`] 和 xref:reference/concurrent_flat_map.adoc#concurrent_flat_map[`boost::concurrent_flat_map]` for the complete list of visitation-enabled operations.++_++flat++_++map`] 的参考文档。
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Although expected to be used much less frequently, concurrent containers also provide insertion operations where an element can be visited right after element creation (in addition to the usual visitation when an equivalent element already exists):
Although expected to be used much less frequently, concurrent containers also provide insertion operations where an element can be visited right after element creation (in addition to the usual visitation when an equivalent element already exists):尽管预期使用频率较低，并发容器还提供在元素创建后立即进行访问的插入操作（除在已存在等价元素时执行常规访问之外）：
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Note that in this last example the visitation function could actually _modify_ the element: as a general rule, operations on a concurrent map `m` will grant visitation functions const/non-const access to the element depending on whether `m` is const/non-const. Const access can be always be explicitly requested by using `cvisit` overloads (for instance, `insert_or_cvisit`) and may result in higher parallelization. For concurrent sets, on the other hand, visitation is always const access.
Note that in this last example the visitation function could actually _modify_ the element: as a general rule, operations on a concurrent map `m` will grant visitation functions const/non-const access to the element depending on whether `m` is const/non-const. Const access can be always be explicitly requested by using `cvisit` overloads (for instance, `insert_or_cvisit`) and may result in higher parallelization. For concurrent sets, on the other hand, visitation is always const access.注意此例中访问函数可__修改__元素：作为通用规则，对并发映射 `m` 的操作将根据 `m` 是否为常量类型，授予访问函数对元素的常量/非常量访问权限。 通过使用 `cvisit` 重载（如 `insert++_++or++_++cvisit` ）可始终显式请求常量访问，这可能提升并行性。而并发集合的访问始终为常量访问。
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Visitation is prominent in the API provided by concurrent containers, and many classical operations have visitation-enabled variations:
Visitation is prominent in the API provided by concurrent containers, and many classical operations have visitation-enabled variations:访问机制在并发容器 API 中占据核心地位，许多经典操作都提供了支持访问机制的变体：
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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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This interoperability comes handy in multistage scenarios where parts of the data processing happen in parallel whereas other steps are non-concurrent (or non-modifying). In the following example, we want to construct a histogram from a huge input vector of words: the population phase can be done in parallel with `boost::concurrent_flat_map` and results then transferred to the final container.
This interoperability comes handy in multistage scenarios where parts of the data processing happen in parallel whereas other steps are non-concurrent (or non-modifying). In the following example, we want to construct a histogram from a huge input vector of words: the population phase can be done in parallel with `boost::concurrent_flat_map` and results then transferred to the final container.
06/05/2026

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