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# std::map

From cppreference.com
< cpp‎ | container

Defined in header `<map>` |  |
template< class Key, class T, class Compare = std::less<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > class map; | (1) |
namespace pmr { template <class Key, class T, class Compare = std::less<Key>> using map = std::map<Key, T, Compare, std::pmr::polymorphic_allocator<std::pair<const Key,T>>> } | (2) | (since C++17)

`std::map` is a sorted associative container that contains key-value pairs with unique keys. Keys are sorted by using the comparison function `Compare`. Search, removal, and insertion operations have logarithmic complexity. Maps are usually implemented as red-black trees.

Everywhere the standard library uses the `Compare` concept, uniqueness is determined by using the equivalence relation. In imprecise terms, two objects `a` and `b` are considered equivalent (not unique) if neither compares less than the other: `!comp(a, b) && !comp(b, a)`.

`std::map` meets the requirements of `Container`, `AllocatorAwareContainer`, `AssociativeContainer` and `ReversibleContainer`.

### [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]
`key_compare` | `Compare` [edit]
`allocator_type` | `Allocator` [edit]
`reference` |
`Allocator::reference` | (until C++11)
`value_type&` | (since C++11)
[edit]
`const_reference` |
`Allocator::const_reference` | (until C++11)
`const value_type&` | (since C++11)
[edit]
`pointer` |
`Allocator::pointer` | (until C++11)
std::allocator_traits<Allocator>::pointer | (since C++11)
[edit]
`const_pointer` |
`Allocator::const_pointer` | (until C++11)
std::allocator_traits<Allocator>::const_pointer | (since C++11)
[edit]
`iterator` | `BidirectionalIterator` [edit]
`const_iterator` | Constant bidirectional iterator [edit]
`reverse_iterator` | std::reverse_iterator<iterator> [edit]
`const_reverse_iterator` | std::reverse_iterator<const_iterator> [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 classes

value_compare | compares objects of type `value_type` (class) [edit]

### [edit] Member functions

(constructor) | constructs the `map` (public member function) [edit]
(destructor) | destructs the `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]
##### Element access
at (C++11) | access specified element with bounds checking (public member function) [edit]
operator[] | access specified element (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]
rbegin crbegin | returns a reverse iterator to the beginning (public member function) [edit]
rend crend | returns a reverse 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 (C++11) | constructs element in-place (public member function) [edit]
emplace_hint (C++11) | 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
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]
lower_bound | returns an iterator to the first element not less than the given key (public member function) [edit]
upper_bound | returns an iterator to the first element greater than the given key (public member function) [edit]
##### Observers
key_comp | returns the function that compares keys (public member function) [edit]
value_comp | returns the function that compares keys in objects of type value_type (public member function) [edit]

### [edit] Non-member functions

operator==operator!=operator<operator<=operator>operator>= | lexicographically compares the values in the map (function template) [edit]
std::swap(std::map) | specializes the std::swap algorithm (function template) [edit]

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