en	:idprefix: hash_equality_

en	= Equality Predicates and Hash Functions
zh-hans	= 相等性谓词与哈希函数

en	While the associative containers use an ordering relation to specify how the elements are stored, the unordered associative containers use an equality predicate and a hash function. For example, `xref:reference/unordered_map.adoc[boost::unordered_map]` is declared as:
zh-hans	关联容器使用排序关系来规定元素的存储方式，而无序关联容器则依赖相等性谓词与哈希函数。例如， `xref:reference/unordered_map.adoc[boost::unordered_map]`的声明如下：

en	```cpp template < class Key, class Mapped, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<Key const, Mapped> > > class unordered_map; ```
zh-hans	```cpp template &lt; class Key, class Mapped, class Hash = boost::hash<key>, class Pred = std::equal_to<key>, class Alloc = std::allocator<std::pair<key const,="" mapped=""> &gt; &gt; class unordered_map; ```</std::pair<key></key></key>

en	The hash function comes first as you might want to change the hash function but not the equality predicate. For example, if you wanted to use the https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function#FNV-1a_hash[FNV-1a hash^] you could write:
zh-hans	哈希函数放在首位，因为你可能只想更改哈希函数而不想更改相等谓词。例如，如果你想使用 https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function#FNV-1a_hash[FNV-1a 哈希^]，可以这样写：

en	```cpp boost::unordered_map<std::string, int, hash::fnv_1a> dictionary; ```
zh-hans	```cpp boost::unordered_map<std::string, int,="" hash::fnv_1a=""> dictionary; ```</std::string,>

en	There is an link:../../../examples/fnv1.hpp[implementation of FNV-1a^] in the examples directory.
zh-hans	examples 目录中有一个链接：../../../examples/fnv1.hpp[implementation of FNV-1a^]

en	If you wish to use a different equality function, you will also need to use a matching hash function. For example, to implement a case insensitive dictionary you need to define a case insensitive equality predicate and hash function:
zh-hans	```cpp

en	```cpp struct iequal_to { bool operator()(std::string const& x, std::string const& y) const { return boost::algorithm::iequals(x, y, std::locale()); } };
zh-hans	```cpp struct iequal_to { bool operator()(std::string const&amp; x, std::string const&amp; y) const { return boost::algorithm::iequals(x, y, std::locale()); } };

en	struct ihash { std::size_t operator()(std::string const& x) const { std::size_t seed = 0; std::locale locale;
zh-hans	struct ihash { std::size_t operator()(std::string const&amp; x) const { std::size_t seed = 0; std::locale locale;

en	for(std::string::const_iterator it = x.begin(); it != x.end(); ++it) { boost::hash_combine(seed, std::toupper(*it, locale)); }

en	return seed; } }; ```

en	Which you can then use in a case insensitive dictionary: ```cpp boost::unordered_map<std::string, int, ihash, iequal_to> idictionary; ```
zh-hans	然后你可以在不区分大小写的字典中使用它：```cpp boost::unordered_map<std::string, int,="" ihash,="" iequal_to=""> idictionary; ```</std::string,>

en	This is a simplified version of the example at link:../../../examples/case_insensitive.hpp[/libs/unordered/examples/case_insensitive.hpp^] which supports other locales and string types.
zh-hans	这是 link:../../../examples/case_insensitive.hpp[/libs/unordered/examples/case_insensitive.hpp^] 处示例的简化版本，该示例支持其他区域设置和字符串类型。

en	Be careful when using the equality (`==`) operator with custom equality
zh-hans	在使用自定义相等谓词时，请谨慎使用相等（==）运算符

en	predicates, especially if you're using a function pointer. If you compare two containers with different equality predicates then the result is undefined. For most stateless function objects this is impossible - since you can only compare objects with the same equality predicate you know the equality predicates must be equal. But if you're using function pointers or a stateful equality predicate (e.g. `boost::function`) then you can get into trouble.
zh-hans	在使用自定义相等谓词（特别是函数指针）时请务必小心。如果你用不同的相等谓词比较两个容器，结果是未定义的。对于大多数无状态函数对象而言，这种情况不可能发生——因为你只能使用相同的相等谓词来比较对象，所以两个相等谓词必然相等。但如果你使用的是函数指针或有状态的相等谓词（例如 boost::function），则可能会遇到问题。

en	Custom Types
zh-hans	自定义类型

en	Similarly, a custom hash function can be used for custom types:
zh-hans	类似地，自定义类型也可使用自定义哈希函数：

en	```cpp struct point { int x; int y; };

en	bool operator==(point const& p1, point const& p2) { return p1.x == p2.x && p1.y == p2.y; }
zh-hans	bool operator==(point const&amp; p1, point const&amp; p2) { return p1.x == p2.x &amp;&amp; p1.y == p2.y; }

en	struct point_hash { std::size_t operator()(point const& p) const { std::size_t seed = 0; boost::hash_combine(seed, p.x); boost::hash_combine(seed, p.y); return seed; } };
zh-hans	struct point_hash { std::size_t operator()(point const&amp; p) const { std::size_t seed = 0; boost::hash_combine(seed, p.x); boost::hash_combine(seed, p.y); return seed; } };

en	boost::unordered_multiset<point, point_hash> points; ```
zh-hans	boost::unordered_multiset<point, point_hash=""> points; ```</point,>

en	Since the default hash function is link:../../../../container_hash/index.html[Boost.Hash^], we can extend it to support the type so that the hash function doesn't need to be explicitly given:
zh-hans	由于默认哈希函数是 link:../../../../container_hash/index.html[Boost.Hash^]，我们可以扩展它以支持该类型，从而无需显式给出哈希函数：

en	```cpp struct point { int x; int y; };

en	bool operator==(point const& p1, point const& p2) { return p1.x == p2.x && p1.y == p2.y; }
zh-hans	bool operator==(point const&amp; p1, point const&amp; p2) { return p1.x == p2.x &amp;&amp; p1.y == p2.y; }

en	std::size_t hash_value(point const& p) { std::size_t seed = 0; boost::hash_combine(seed, p.x); boost::hash_combine(seed, p.y); return seed; }
zh-hans	std::size_t hash_value(point const&amp; p) { std::size_t seed = 0; boost::hash_combine(seed, p.x); boost::hash_combine(seed, p.y); return seed; }

en	boost::unordered_multiset<point> points; ```
zh-hans	boost::unordered_multiset<point> points; ```</point>

en	See the link:../../../../container_hash/index.html[Boost.Hash documentation^] for more detail on how to do this. Remember that it relies on extensions to the standard - so it won't work for other implementations of the unordered associative containers, you'll need to explicitly use Boost.Hash.
zh-hans	有关如何执行此操作的更多详细信息，请参阅 link:../../../../container_hash/index.html[Boost.Hash 文档^]。请记住，它依赖于标准扩展—因此它不适用于无序关联容器的其他实现，你需要显式使用 Boost.Hash。

en	Method
zh-hans	方法

en	Description
zh-hans	描述

en	`hasher hash_function() const`

en	Returns the container's hash function.
zh-hans	返回容器的哈希函数。

en	`key_equal key_eq() const`

en	Returns the container's key equality function..
zh-hans	返回容器的键相等性函数。