2015-03-09 22:17:42 閱讀：
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unordered_map和map類似，都是儲存的key-value的值，可以通過key快速索引到value。
不同的是unordered_map不會根據key的大小進行排序，儲存時是根據key的hash值判斷元素是否相同，即unordered_map內部元素是無序的，而map中的元素是按照二叉搜尋樹儲存，進行中序遍歷會得到有序遍歷。
所 以使用時map的key需要定義operator<。而unordered_map需要定義hash_value函式並且過載 operator==。但是很多系統內建的資料型別都自帶這些，那麼如果是自定義型別，那麼就需要自己過載operator《或者 hash_value()了。
結論：如果需要內部元素自動排序，使用map，不需要排序使用unordered_map
c++0x為什麼不把unordered_map定義為hash_map呢？那是因為在新標準出現之前很多庫廠商已經暫用了hash_map這個名詞。因此為了向前相容不得不定義新的unordered_map。
函式原型
template < class key, // unordered_map::key_type(constructor)class hash = hash, // unordered_map::hasher
class pred = equal_to, // unordered_map::key_equal
class alloc = allocator< pair> // unordered_map::allocator_type
>
class unordered_map;
key表示建的型別
t表示鍵對映到的hash值的型別
hash是乙個接受乙個引數且型別要與key相容的函式物件。返回值為t型。注意，引數是const引用， 函式是const
pred是乙個接受兩個引數且其型別與key相容的函式物件。返回值為bool型。注意，引數是const引用， 函式是const
成員函式：
construct
unordered_map
(public member function )
(destructor)
destroy unordered map
(public member function)
operator=
assign content
(public member function )
capacityempty
test whether container is empty
(public member function)
size
return container size
(public member function)
max_size
return maximum size
(public member function)
iteratorsbegin
return iterator to beginning
(public member function)
endreturn 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 accessoperator
access element
(public member function ) at
access element
(public member function)
element lookupfind
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)
modifiersemplace
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)
bucketsbucket_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 policyload_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)
observershash_function
get hash function
(public member type)
key_eq
get key equivalence predicate
(public member type)
get_allocator
get allocator
(public member function)
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