New features in c++11: unordered_map

Unordered_map, like map, are stored key-value values that can be quickly indexed to value by key.

The difference is that the unordered_map will not be sorted according to the size of the key, the storage is based on the hash value of key to determine whether the element is the same, that is, unordered_map internal elements are unordered, and map elements are stored according to the binary search tree, The sequential traversal is carried out sequentially.

So when using the key of map, we need to define operator<. Instead, Unordered_map needs to define the Hash_value function and overload the operator==. But many of the system's built-in data types come with these, so if it's a custom type, you'll need to reload operator< or Hash_value () yourself.

Conclusion: If you need to sort internal elements automatically, use map and do not need to sort using unordered_map

c++0x why not define unordered_map as Hash_map? That's because many library manufacturers have taken up the term hash_map before the new standard comes into being. Therefore, the new unordered_map must be defined for forward compatibility.

Function prototypes

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 indicates the type of build
T denotes the type of hash value to which the key is mapped
A hash is a function object that takes a parameter and the type is compatible with the key. The return value is type T. Note that the parameter is a const reference and the function is a const
Pred is a function object that accepts two parameters and has a type that is compatible with key. The return value is type bool. Note that the parameter is a const reference and the function is a const




member functions:

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)

Buckets

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)

New features in c++11: unordered_map