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open addressing degrades badly when there are a high number of collisions the restrictions could prevent a rehash when it's really needed. The maximum load factor could be set to a fairly low value to work around this - but the standard requires that it is initially set to 1.0.
open addressing degrades badly when there are a high number of collisions the restrictions could prevent a rehash when it's really needed. The maximum load factor could be set to a fairly low value to work around this - but the standard requires that it is initially set to开放寻址法在发生大量冲突时性能会急剧下降，这些限制可能会阻止必要的重新哈希操作。虽然可以通过设置较低的最大负载因子来规避此问题——但标准要求该值初始必须设为 1.0.。
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There are also the restrictions on when iterators can be invalidated. Since
There are also the restrictions on when iterators can be invalidated. Since此外，标准对迭代器失效的时机也存在限制。由于
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meet the complexity requirements.
meet the complexity requirements.满足复杂度要求。
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Local iterators would be very inefficient and may not be able to
Local iterators would be very inefficient and may not be able to局部迭代器的实现将极为低效，且可能无法
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the elements can't be stored in one array, but will need a layer of indirection instead - losing the efficiency and most of the memory gain, the main advantages of open addressing.
the elements can't be stored in one array, but will need a layer of indirection instead - losing the efficiency and most of the memory gain, the main advantages of open addressing.元素无法存储在一个连续的数组中，而需要增加一层间接寻址——这将损失开放定址法的主要优势，即效率和大部分内存收益。
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The standard requires that pointers to elements aren't invalidated, so
The standard requires that pointers to elements aren't invalidated, so该标准要求指向元素的指针不得失效，因此
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It would be conceivable to write a hash table that uses another method. For example, it could use open addressing, and use the lookup chain to act as a bucket but there are some serious problems with this:
It would be conceivable to write a hash table that uses another method. For example, it could use open addressing, and use the lookup chain to act as a bucket but there are some serious problems with this:理论上完全可以设计采用其他实现方式的哈希表。例如，采用开放寻址法，利用查找链来模拟桶的行为，但这种做法会带来一些严重问题：
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By specifying an interface for accessing the buckets of the container the standard pretty much requires that the hash table uses closed addressing.
By specifying an interface for accessing the buckets of the container the standard pretty much requires that the hash table uses closed addressing.通过制定用于访问容器桶的接口，该标准实质上要求哈希表必须采用闭寻址方案。
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Data Structure
Data Structure数据结构
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The http://en.wikipedia.org/wiki/Hash_table[Wikipedia article on hash tables^] has a good summary of the implementation issues for hash tables in general.
The关于哈希表的通用实现问题的详细综述，可参阅 http://en.wikipedia.org/wiki/Hash_table[Wikipedia article on hash tables^] has a good summary of the implementation issues for hash tables in general.维基百科哈希表条目] 。
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`boost::unordered_[multi]set` and `boost::unordered_[multi]map` adhere to the standard requirements for unordered associative containers, so the interface was fixed. But there are still some implementation decisions to make. The priorities are conformance to the standard and portability.
`boost::unordered_[multi]++_[++multi++]++set` and与 `boost::unordered_[multi]map` adhere to the standard requirements for unordered associative containers, so the interface was fixed. But there are still some implementation decisions to make. The priorities are conformance to the standard and portability.++_[++multi++]++map` 遵循无序关联容器的标准规范，因此其接口是确定的。但在实现层面仍需做出一些决策，其首要考量是标准符合性与跨平台移植性。
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Closed-addressing Containers
Closed-addressing Containers闭寻址容器
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= Implementation Rationale
= Implementation Rationale实现设计依据
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:idprefix: rationale_
:idprefix: rationale_
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Storing an object in an unordered associative container requires both a key equality function and a hash function. The default function objects in the standard containers support a few basic types including integer types, floating point types, pointer types, and the standard strings. Since Boost.Unordered uses link:../../../../container_hash/index.html[boost::hash^] it also supports some other types, including standard containers. To use any types not supported by these methods you have to extend Boost.Hash to support the type or use your own custom equality predicates and hash functions. See the xref:hash_equality.adoc#hash_equality[Equality Predicates and Hash Functions], section for more details.
Storing an object in an unordered associative container requires both a key equality function and a hash function. The default function objects in the standard containers support a few basic types including integer types, floating point types, pointer types, and the standard strings. Since在无序关联容器中存储对象需要同时提供键相等函数和哈希函数。标准容器中的默认函数对象支持若干基本类型，包括整数类型、浮点类型、指针类型以及标准字符串。由于 Boost.Unordered uses使用 link:../../../../container_hash/index.html[boost::hash^] it also supports some other types, including standard containers. To use any types not supported by these methods you have to extend Boost.Hash to support the type or use your own custom equality predicates and hash functions. See the xref:hash_equality.adoc#hash_equality[Equality Predicates and Hash Functions], section for more details.，它还支持其他一些类型，包括标准容器。要使用这些方法不支持的任意类型，用户必须扩展 Boost.Hash 以支持该类型，或者使用自定义的相等谓词和哈希函数。更多详情请参见 xref:hash_equality.adoc#hash_equality[相等谓词与哈希函数] 一节。
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All sets and maps in Boost.Unordered are instantiatied similarly as `std::unordered_set` and `std::unordered_map`, respectively:
All sets and maps in Boost.Unordered are instantiatied similarly asBoost.Unordered 中的所有 set 和 map 分别与 `std::unordered_set` and和 `std::unordered_map`, respectively: 类似的方式进行实例化：
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multithreaded scenarios. Their interface is radically different from that of regular C++ containers. Flat and node-based variants are provided.
multithreaded scenarios. Their interface is radically different from that of regular C++ containers. Flat and node-based variants are provided.其接口与常规C{plus}{plus}容器存在根本性差异。该系列同时提供扁平与基于节点两种变体。
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Finally, **concurrent containers** are designed and implemented to be used in high-performance
Finally, **concurrent containers** are designed and implemented to be used in high-performance最后，**并发容器**专为高性能多线程场景设计与实现，
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(more than 2 times faster in typical scenarios) while slightly diverging from the standard interface to accommodate the implementation. There are two variants: **flat** (the fastest) and **node-based**, which provide pointer stability under rehashing at the expense of being slower.
(more than 2 times faster in typical scenarios) while slightly diverging from the standard interface to accommodate the implementation. There are two variants: **flat** (the fastest) and **node-based**, which provide pointer stability under rehashing at the expense of being slower.（典型场景下性能提升2倍以上），同时为了适配实现方案而对标准接口做了细微调整。该类容器包含两种变体：*扁平式*（最快）与**基于节点式**，后者在重新哈希时能保持指针稳定性，但性能会有所下降。
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