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# std::unordered_map

From cppreference.com
< cpp‎ | container

Defined in header `<unordered_map>` |  |
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)

Unordered map is an associative container that contains key-value pairs with unique keys. Search, insertion, and removal of elements have average constant-time complexity.

Internally, the elements are not sorted in any particular order, but organized into buckets. Which bucket an element is placed into depends entirely on the hash of its key. This allows fast access to individual elements, since once the hash is computed, it refers to the exact bucket the element is placed into.

`std::unordered_map` meets the requirements of `Container`, `AllocatorAwareContainer`, `UnorderedAssociativeContainer`.

### [edit] Iterator invalidation

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

#### [edit] Notes

- The swap functions do not invalidate any of the iterators inside the container, but they do invalidate the iterator marking the end of the swap region.

- References and pointers to either key or data stored in the container are only invalidated by erasing that element, even when the corresponding iterator is invalidated.

### [edit] Member types

Member type | Definition
`key_type` | `Key` [edit]
`mapped_type` | `T` [edit]
`value_type` | std::pair<const Key, T> [edit]
`size_type` | Unsigned integer type (usually std::size_t) [edit]
`difference_type` | Signed integer type (usually std::ptrdiff_t) [edit]
`hasher` | `Hash` [edit]
`key_equal` | `KeyEqual` [edit]
`allocator_type` | `Allocator` [edit]
`reference` | `value_type&` [edit]
`const_reference` | `const value_type&` [edit]
`pointer` | std::allocator_traits<Allocator>::pointer [edit]
`const_pointer` | std::allocator_traits<Allocator>::const_pointer [edit]
`iterator` | `ForwardIterator` [edit]
`const_iterator` | Constant forward iterator [edit]
`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[edit]
`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[edit]
`node_type` | a specialization of node handle representing a container node (since C++17) [edit]
`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) [edit]

### [edit] Member functions

(constructor) | constructs the `unordered_map` (public member function) [edit]
(destructor) | destructs the `unordered_map` (public member function) [edit]
operator= | assigns values to the container (public member function) [edit]
get_allocator | returns the associated allocator (public member function) [edit]
##### Iterators
begin cbegin | returns an iterator to the beginning (public member function) [edit]
end cend | returns an iterator to the end (public member function) [edit]
##### Capacity
empty | checks whether the container is empty (public member function) [edit]
size | returns the number of elements (public member function) [edit]
max_size | returns the maximum possible number of elements (public member function) [edit]
##### Modifiers
clear | clears the contents (public member function) [edit]
insert | inserts elements or nodes (since C++17) (public member function) [edit]
insert_or_assign (C++17) | inserts an element or assigns to the current element if the key already exists (public member function) [edit]
emplace | constructs element in-place (public member function) [edit]
emplace_hint | constructs elements in-place using a hint (public member function) [edit]
try_emplace (C++17) | inserts in-place if the key does not exist, does nothing if the key exists (public member function) [edit]
erase | erases elements (public member function) [edit]
swap | swaps the contents (public member function) [edit]
extract (C++17) | extracts nodes from the container (public member function) [edit]
merge (C++17) | splices nodes from another container (public member function) [edit]
##### Lookup
at | access specified element with bounds checking (public member function) [edit]
operator[] | access specified element (public member function) [edit]
count | returns the number of elements matching specific key (public member function) [edit]
find | finds element with specific key (public member function) [edit]
equal_range | returns range of elements matching a specific key (public member function) [edit]
##### Bucket interface
begin(int) cbegin(int) | returns an iterator to the beginning of the specified bucket (public member function) [edit]
end(int) cend(int) | returns an iterator to the end of the specified bucket (public member function) [edit]
bucket_count | returns the number of buckets (public member function) [edit]
max_bucket_count | returns the maximum number of buckets (public member function) [edit]
bucket_size | returns the number of elements in specific bucket (public member function) [edit]
bucket | returns the bucket for specific key (public member function) [edit]
##### Hash policy
load_factor | returns average number of elements per bucket (public member function) [edit]
max_load_factor | manages maximum average number of elements per bucket (public member function) [edit]
rehash | reserves at least the specified number of buckets. This regenerates the hash table. (public member function) [edit]
reserve | reserves space for at least the specified number of elements. This regenerates the hash table. (public member function) [edit]
##### Observers
hash_function | returns function used to hash the keys (public member function) [edit]
key_eq | returns the function used to compare keys for equality (public member function) [edit]

### [edit] Non-member functions

operator==operator!= | compares the values in the unordered_map (function template) [edit]
std::swap(std::unordered_map) (C++11) | specializes the std::swap algorithm (function template) [edit]

### [edit] Example

Run this code

    #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;
    }

Output:

    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]

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