关联性容器

关联容器是指C++标准模板库中的一套类模板，实现了有序关联数组。可用于存放任意数据类型的元素。C++标准中定义的关联容器有： `set`, `map`, `multiset`, `multimap`。

关联容器类似于C++中的无序关联容器。差别为：

- 关联容器是红黑树实现，无序关联容器是哈希表实现。
- 关联容器保证按键值有序遍历，因此可以做范围查找，而无序关联容器不可以。关联支持一些导航类的操作，如求出给定键最邻近的键，最大键、最小键操作。
- 关联容器的迭代器不会失效，除非所指元素被删除。无序关联容器的iterator在修改元素时可能会失效。所以对关联容器的遍历与修改在一定程度上可并行
- 哈希表查找时候要算hash，这个最坏时间复杂度是O(key的长度)；基于比较的有序关联容器通常只使用头几个字符进行比较

## 成员函数

`set`｜`map`｜`multiset`｜`multimap`｜描述

(constructor)｜ (constructor)｜ (constructor)｜ (constructor)｜构造

(destructor)｜ (destructor)｜ (destructor)｜ (destructor)｜析构

`[http://en.cppreference.com/w/cpp/container/set/operator= operator=]`｜`[http://en.cppreference.com/w/cpp/container/map/operator= operator=]`｜`[http://en.cppreference.com/w/cpp/container/multiset/operator= operator=]`｜`[http://en.cppreference.com/w/cpp/container/multimap/operator= operator=]`｜容器赋值

`[http://en.cppreference.com/w/cpp/container/set/get_allocator get_allocator]`｜`[http://en.cppreference.com/w/cpp/container/map/get_allocator get_allocator]`｜`[http://en.cppreference.com/w/cpp/container/multiset/get_allocator get_allocator]`｜`[http://en.cppreference.com/w/cpp/container/multimap/get_allocator get_allocator]`｜返回分配器，用于给容器的成员分配空间

Element access｜不适用｜`[http://en.cppreference.com/w/cpp/container/map/at at]`｜不适用｜不适用｜访问特定元素，带边界检查

不适用｜ operator`[]`｜不适用｜不适用｜访问特定元素，不带边界检查

Iterators｜`[http://en.cppreference.com/w/cpp/container/set/begin begin]`｜`[http://en.cppreference.com/w/cpp/container/map/begin begin]`｜`[http://en.cppreference.com/w/cpp/container/multiset/begin begin]`｜`[http://en.cppreference.com/w/cpp/container/multimap/begin begin]`｜返回容器开始处的迭代器

`[http://en.cppreference.com/w/cpp/container/set/end end]`｜`[http://en.cppreference.com/w/cpp/container/map/end end]`｜`[http://en.cppreference.com/w/cpp/container/multiset/end end]`｜`[http://en.cppreference.com/w/cpp/container/multimap/end end]`｜返回容器结束处的迭代器

`[http://en.cppreference.com/w/cpp/container/set/rbegin rbegin]`｜`[http://en.cppreference.com/w/cpp/container/map/rbegin rbegin]`｜`[http://en.cppreference.com/w/cpp/container/multiset/rbegin rbegin]`｜`[http://en.cppreference.com/w/cpp/container/multimap/rbegin rbegin]`｜返回容器逆向开始处的逆向迭代器

`[http://en.cppreference.com/w/cpp/container/set/rend rend]`｜`[http://en.cppreference.com/w/cpp/container/map/rend rend]`｜`[http://en.cppreference.com/w/cpp/container/multiset/rend rend]`｜`[http://en.cppreference.com/w/cpp/container/multimap/rend rend]`｜返回容器逆向结束处的逆向迭代器

Capacity｜`[http://en.cppreference.com/w/cpp/container/set/empty empty]`｜`[http://en.cppreference.com/w/cpp/container/map/empty empty]`｜`[http://en.cppreference.com/w/cpp/container/multiset/empty empty]`｜`[http://en.cppreference.com/w/cpp/container/multimap/empty empty]`｜检查容器是否为空

`[http://en.cppreference.com/w/cpp/container/set/size size]`｜`[http://en.cppreference.com/w/cpp/container/map/size size]`｜`[http://en.cppreference.com/w/cpp/container/multiset/size size]`｜`[http://en.cppreference.com/w/cpp/container/multimap/size size]`｜返回容器内元素数量

`[http://en.cppreference.com/w/cpp/container/set/max_size max_size]`｜`[http://en.cppreference.com/w/cpp/container/map/max_size max_size]`｜`[http://en.cppreference.com/w/cpp/container/multiset/max_size max_size]`｜`[http://en.cppreference.com/w/cpp/container/multimap/max_size max_size]`｜返回容器内元素最大可能处理

Modifiers｜`[http://en.cppreference.com/w/cpp/container/set/clear clear]`｜`[http://en.cppreference.com/w/cpp/container/map/clear clear]`｜`[http://en.cppreference.com/w/cpp/container/multiset/clear clear]`｜`[http://en.cppreference.com/w/cpp/container/multimap/clear clear]`｜清空容器内容

`[http://en.cppreference.com/w/cpp/container/set/insert insert]`｜`[http://en.cppreference.com/w/cpp/container/map/insert insert]`｜`[http://en.cppreference.com/w/cpp/container/multiset/insert insert]`｜`[http://en.cppreference.com/w/cpp/container/multimap/insert insert]`｜插入元素

`[http://en.cppreference.com/w/cpp/container/set/emplace emplace]`｜`[http://en.cppreference.com/w/cpp/container/map/emplace emplace]`｜`[http://en.cppreference.com/w/cpp/container/multiset/emplace emplace]`｜`[http://en.cppreference.com/w/cpp/container/multimap/emplace emplace]`｜原地构造元素 (C++11)

`[http://en.cppreference.com/w/cpp/container/set/emplace_hint emplace_hint]`｜`[http://en.cppreference.com/w/cpp/container/map/emplace_hint emplace_hint]`｜`[http://en.cppreference.com/w/cpp/container/multiset/emplace_hint emplace_hint]`｜`[http://en.cppreference.com/w/cpp/container/multimap/emplace_hint emplace_hint]`｜使用hint 原地构造元素(C++11)

`[http://en.cppreference.com/w/cpp/container/set/erase erase]`｜`[http://en.cppreference.com/w/cpp/container/map/erase erase]`｜`[http://en.cppreference.com/w/cpp/container/multiset/erase erase]`｜`[http://en.cppreference.com/w/cpp/container/multimap/erase erase]`｜擦除元素

`[http://en.cppreference.com/w/cpp/container/set/swap swap]`｜`[http://en.cppreference.com/w/cpp/container/map/swap swap]`｜`[http://en.cppreference.com/w/cpp/container/multiset/swap swap]`｜`[http://en.cppreference.com/w/cpp/container/multimap/swap swap]`｜与另一个容器交换内容

Lookup｜`[http://en.cppreference.com/w/cpp/container/set/count count]`｜`[http://en.cppreference.com/w/cpp/container/map/count count]`｜`[http://en.cppreference.com/w/cpp/container/multiset/count count]`｜`[http://en.cppreference.com/w/cpp/container/multimap/count count]`｜返回匹配特点键值的元素数量

`[http://en.cppreference.com/w/cpp/container/set/find find]`｜`[http://en.cppreference.com/w/cpp/container/map/find find]`｜`[http://en.cppreference.com/w/cpp/container/multiset/find find]`｜`[http://en.cppreference.com/w/cpp/container/multimap/find find]`｜搜到特定键值的元素

`[http://en.cppreference.com/w/cpp/container/set/equal_range equal_range]`｜`[http://en.cppreference.com/w/cpp/container/map/equal_range equal_range]`｜`[http://en.cppreference.com/w/cpp/container/multiset/equal_range equal_range]`｜`[http://en.cppreference.com/w/cpp/container/multimap/equal_range equal_range]`｜返回匹配特定键值的所有元素

`[http://en.cppreference.com/w/cpp/container/set/lower_bound lower_bound]`｜`[http://en.cppreference.com/w/cpp/container/map/lower_bound lower_bound]`｜`[http://en.cppreference.com/w/cpp/container/multiset/lower_bound lower_bound]`｜`[http://en.cppreference.com/w/cpp/container/multimap/lower_bound lower_bound]`｜返回键值不小于特定值的第一个元素的迭代器

`[http://en.cppreference.com/w/cpp/container/set/upper_bound upper_bound]`｜`[http://en.cppreference.com/w/cpp/container/map/upper_bound upper_bound]`｜`[http://en.cppreference.com/w/cpp/container/multiset/upper_bound upper_bound]`｜`[http://en.cppreference.com/w/cpp/container/multimap/upper_bound upper_bound]`｜返回键值大于特定值的第一个元素的迭代器

Observers｜`[http://en.cppreference.com/w/cpp/container/set/key_comp key_comp]`｜`[http://en.cppreference.com/w/cpp/container/map/key_comp key_comp]`｜`[http://en.cppreference.com/w/cpp/container/multiset/key_comp key_comp]`｜`[http://en.cppreference.com/w/cpp/container/multimap/key_comp key_comp]`｜返回键值比较函数

`[http://en.cppreference.com/w/cpp/container/set/value_comp value_comp]`｜`[http://en.cppreference.com/w/cpp/container/map/value_comp value_comp]`｜`[http://en.cppreference.com/w/cpp/container/multiset/value_comp value_comp]`｜`[http://en.cppreference.com/w/cpp/container/multimap/value_comp value_comp]`｜返回值比较函数。在`set`与`multiset`，该函数equivalent to `key_comp`, 因为元素只有值.

## 用法

下述例子展示如何用`map<string, int>`计数word的次数。用word为键值，次数为值。

```cpp
#include <iostream>
#include <string>
#include <map>

int main()
{
    std::map<std::string, int> wordcounts;
    std::string s;

    while (std::cin >> s && s != "end")
        ++wordcounts[s];

    while (std::cin >> s && s != "end")
        std::cout << s << ' ' << wordcounts[s] << '\n';
}
```

执行时，用户线输入一系列word，最后以"end"结束输入。然后输入word可查询它出现的次数。

下例展示用insert函数、find函数：

```cpp
#include <iostream>
#include <map>
#include <utility> // make_pair

int main()
{
    typedef std::map<char, int> MapType;
    MapType my_map;

    // insert elements using insert function
    my_map.insert(std::pair<char, int>('a', 1));
    my_map.insert(std::pair<char, int>('b', 2));
    my_map.insert(std::pair<char, int>('c', 3));
    my_map.insert(MapType::value_type('d', 4)); // all standard containers provide this typedef
    my_map.insert(std::make_pair('e', 5));      // can also use the utility function make_pair
    my_map.insert({'f', 6});                    // using C++11 initializer list
    
    //map keys are sorted automatically from lower to higher. 
    //So, my_map.begin() points to the lowest key value not the key which was inserted first.
    MapType::iterator iter = my_map.begin();

    // erase the first element using the erase function
    my_map.erase(iter);

    // output the size of the map
    std::cout << "Size of my_map: " << my_map.size() << '\n';

    std::cout << "Enter a key to search for: ";
    char c;
    std::cin >> c;

    // find will return an iterator to the matching element if it is found
    // or to the end of the map if the key is not found
    iter = my_map.find(c);
    if (iter != my_map.end() ) 
        std::cout << "Value is: " << iter->second << '\n';
    else
        std::cout << "Key is not in my_map" << '\n';

    // clear the entries in the map
    my_map.clear();
}
```

### 迭代器

```cpp
map<Key,T>::iterator it; // declares a map iterator
it->first;               // the key value 
it->second;              // the mapped value
(*it);                   // the "element value", which is of type:  pair<const Key,T>
```

```cpp
#include <iostream>
#include <string>
#include <map>

int main()
{
    std::map <std::string, int> data{
     { "Bobs score", 10 },
     { "Martys score", 15 },
     { "Mehmets score", 34 },
     { "Rockys score", 22 },
     { "Rockys score", 23 } /*overwrites the 22 as keys are identical */
    };
    
    // Iterate over the map and print out all key/value pairs.
    for (const auto& element : data)
    {
        std::cout << "Who(key = first): " << element.first;
        std::cout << " Score(value = second): " << element.second << '\n';
    }

    return 0;
}
```