C++11中新特性之：unordered_map

unordered_map和map类似，都是存储的key-value的值，可以通过key快速索引到value。

不同的是unordered_map不会根据key的大小进行排序，存储时是根据key的hash值判断元素是否相同，即unordered_map内部元素是无序的，而map中的元素是按照二叉搜索树存储，进行中序遍历会得到有序遍历。

所 以使用时map的key需要定义operator<。而unordered_map需要定义hash_value函数并且重载 operator==。但是很多系统内置的数据类型都自带这些，那么如果是自定义类型，那么就需要自己重载operator<或者 hash_value()了。

结论：如果需要内部元素自动排序，使用map，不需要排序使用unordered_map

C++0X为什么不把unordered_map定义为hash_map呢？那是因为在新标准出现之前很多库厂商已经暂用了hash_map这个名词。因此为了向前兼容不得不定义新的unordered_map。

函数原型

template < class Key,                                    // unordered_map::key_type
           class T,                                      // unordered_map::mapped_type
           class Hash = hash<Key>,                       // unordered_map::hasher
           class Pred = equal_to<Key>,                   // unordered_map::key_equal
           class Alloc = allocator< pair<const Key,T> >  // unordered_map::allocator_type
           > 
class unordered_map;

Key表示建的类型
T表示键映射到的hash值的类型
Hash是一个接受一个参数且类型要与Key兼容的函数对象。返回值为T型。注意，参数是const引用， 函数是const
Pred是一个接受两个参数且其类型与Key兼容的函数对象。返回值为BOOL型。注意，参数是const引用， 函数是const

成员函数：

Member functions

(constructor)

Construct unordered_map (public member function )

(destructor)

Destroy unordered map (public member function)

operator=

Assign content (public member function )

Capacity

empty

Test whether container is empty (public member function)

size

Return container size (public member function)

max_size

Return maximum size (public member function)

Iterators

begin

Return iterator to beginning (public member function)

end

Return iterator to end (public member function)

cbegin

Return const_iterator to beginning (public member function)

cend

Return const_iterator to end (public member function)

Element access

operator[]

Access element (public member function )

at

Access element (public member function)

Element lookup

find

Get iterator to element (public member function)

count

Count elements with a specific key (public member function )

equal_range

Get range of elements with specific key (public member function)

Modifiers

emplace

Construct and insert element (public member function )

emplace_hint

Construct and insert element with hint (public member function )

insert

Insert elements (public member function )

erase

Erase elements (public member function )

clear

Clear content (public member function )

swap

Swap content (public member function)

Buckets

bucket_count

Return number of buckets (public member function)

max_bucket_count

Return maximum number of buckets (public member function)

bucket_size

Return bucket size (public member type)

bucket

Locate element's bucket (public member function)

Hash policy

load_factor

Return load factor (public member function)

max_load_factor

Get or set maximum load factor (public member function )

rehash

Set number of buckets (public member function )

reserve

Request a capacity change (public member function)

Observers

hash_function

Get hash function (public member type)

key_eq

Get key equivalence predicate (public member type)

get_allocator

Get allocator (public member function)