std::unordered_map
STD：无序[医]地图
| Defined in header | | |
|---|---|---|
| template< class Key, class T, class Hash = std::hash<Key>, class KeyEqual = std::equal_to<Key>, class Allocator = std::allocator< std::pair<const Key, T> > > class unordered_map; | (1) | (since C++11) |
| namespace pmr { template <class Key, class T, class Hash = std::hash<Key>, class Pred = std::equal_to<Key>> using unordered_map = std::unordered_map<Key, T, Hash, Pred, std::pmr::polymorphic_allocator<std::pair<const Key,T>>>; } | (2) | (since C++17) |
无序映射是一个关联容器，包含具有唯一键的键值对。元素的搜索、插入和删除具有平均恒定时间复杂度.
在内部，元素不是按任何特定的顺序排序，而是组织成桶。将元素放入哪个桶完全取决于其键的散列。这允许快速访问单个元素，因为一旦计算了散列，它就会引用元素被放置到的确切桶中。
std::unordered_map满足…的要求Container，，，AllocatorAwareContainer，，，UnorderedAssociativeContainer...
迭代器失效
| Operations | Invalidated |
|---|---|
| All read only operations, swap, std::swap | Never |
| clear, rehash, reserve, operator= | Always |
| insert, emplace, emplace_hint, operator[] | Only if causes rehash |
| erase | Only to the element erased |
注记
- 交换函数不会使容器中的任何迭代器失效，但它们会使标志着交换区域结束的迭代器失效。
- 对存储在容器中的键或数据的引用和指针仅通过擦除该元素而失效，即使在相应的迭代器失效时也是如此。
成员类型
| Member type | Definition |
|---|---|
| key_type | Key |
| mapped_type | T |
| value_type | std::pair<const Key, T> |
| size_type | Unsigned integer type (usually std::size_t) |
| difference_type | Signed integer type (usually std::ptrdiff_t) |
| hasher | Hash |
| key_equal | KeyEqual |
| allocator_type | Allocator |
| reference | value_type& |
| const_reference | const value_type& |
| pointer | std::allocator_traits<Allocator>::pointer |
| const_pointer | std::allocator_traits<Allocator>::const_pointer |
| iterator | ForwardIterator |
| const_iterator | Constant forward iterator |
| local_iterator | An iterator type whose category, value, difference, pointer and reference types are the same as iterator. This iterator can be used to iterate through a single bucket but not across buckets |
| const_local_iterator | An iterator type whose category, value, difference, pointer and reference types are the same as const_iterator. This iterator can be used to iterate through a single bucket but not across buckets |
| node_type | a specialization of node handle representing a container node (since C++17) |
| insert_return_type | type describing the result of inserting a node_type, a specialization of template <class Iterator, class NodeType> struct /*unspecified*/ { Iterator position; bool inserted; NodeType node; }; instantiated with template arguments iterator and node_type. (since C++17) |
成员函数
| (constructor) | constructs the unordered_map (public member function) |
|---|---|
| (destructor) | destructs the unordered_map (public member function) |
| operator= | assigns values to the container (public member function) |
| get_allocator | returns the associated allocator (public member function) |
迭代器
BEGINCBEGIN将迭代器返回到开头%28的公共成员函数%29
End cend将迭代器返回到End%28公共成员函数%29
容量
空检查容器是否为空%28公共成员函数%29
Size返回元素数%28公共成员函数%29
马克斯[医]Size返回元素的最大可能数%28公共成员函数%29
修饰符
清除内容%28公共成员功能%29
插入元素或节点%28，因为C++17%29%28公共成员函数%29
插入[医]或[医]如果键已经存在，则指定%28C++17%29插入一个元素或分配给当前元素
座落[医]提示使用提示%28公共成员函数%29就地构造元素。
试一试[医]嵌入%28C++17%29如果密钥不存在，则插入就地；如果密钥存在%28公共成员函数%29，则不执行任何操作。
擦除元素%28公共成员函数%29
交换交换内容%28公共成员函数%29
提取%28C++17%29从容器中提取节点%28公共成员函数%29
从另一个容器合并%28C++17%29个连接节点%28公共成员函数%29
查找
在访问指定元素时，使用边界检查%28公共成员函数%29
操作者。[]访问指定元素%28公共成员函数%29
Count返回匹配特定键的元素数%28公共成员函数%29
查找具有特定密钥%28公共成员函数%29的查找元素
平等[医]范围返回匹配特定键%28公共成员函数%29的元素的范围
桶接口
开始%28 int%29 cBEGIN%28 int%29返回一个迭代器到指定桶%28公共成员函数%29的开头
End%28 int%29 cend%28 int%29返回一个迭代器到指定桶%28公共成员函数%29的末尾
斗[医]Count返回桶数%28公共成员函数%29
马克斯[医]斗[医]COUNT返回最大桶数%28公共成员函数%29
斗[医]Size返回特定桶%28公共成员函数中的元素数%29
桶返回特定键%28公共成员函数%29的桶
散列策略
负载[医]因子返回每个桶的平均元素数%28公共成员函数%29
马克斯[医]负载[医]因子管理每个桶的最大平均元素数%28公共成员函数%29
重新哈希至少保留指定数量的桶。这将重新生成哈希表。28%公共成员职能%29
储备空间至少保留指定数量的元素。这将重新生成哈希表。28%公共成员职能%29
观察员
散列[医]函数返回函数用于散列键%28公共成员函数%29
键[医]Eq返回用于比较相等性%28公共成员函数%29的键的函数。
非会员职能
| operator==operator!= | compares the values in the unordered_map (function template) |
|---|---|
| std::swap(std::unordered_map) (C++11) | specializes the std::swap algorithm (function template) |
例
二次
#include <iostream>
#include <string>
#include <unordered_map>
int main()
{
// Create an unordered_map of three strings (that map to strings)
std::unordered_map<std::string, std::string> u = {
{"RED","#FF0000"},
{"GREEN","#00FF00"},
{"BLUE","#0000FF"}
};
// Iterate and print keys and values of unordered_map
for( const auto& n : u ) {
std::cout << "Key:[" << n.first << "] Value:[" << n.second << "]\n";
}
// Add two new entries to the unordered_map
u["BLACK"] = "#000000";
u["WHITE"] = "#FFFFFF";
// Output values by key
std::cout << "The HEX of color RED is:[" << u["RED"] << "]\n";
std::cout << "The HEX of color BLACK is:[" << u["BLACK"] << "]\n";
return 0;
}
二次
产出：
二次
Key:[RED] Value:[#FF0000]
Key:[BLUE] Value:[#0000FF]
Key:[GREEN] Value:[#00FF00]
The HEX of color RED is:[#FF0000]
The HEX of color BLACK is:[#000000]
二次
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