# std::map

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< 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 requirements, 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.

### Template parameters

| This section is incomplete Reason: Add descriptions of the template parameters.

### Member types

Member type | Definition
`key_type` | `Key`
`mapped_type` | `T`
`value_type` | std::pair<const Key, T>
`size_type` | Unsigned integer type (usually std::size_t)
`difference_type` | Signed integer type (usually std::ptrdiff_t)
`key_compare` | `Compare`
`allocator_type` | `Allocator`
`reference` | `value_type&`
`const_reference` | const value_type&
`pointer` |
`Allocator::pointer` | (until C++11)
std::allocator_traits<Allocator>::pointer | (since C++11)
`const_pointer` |
`Allocator::const_pointer` | (until C++11)
std::allocator_traits<Allocator>::const_pointer | (since C++11)
`iterator` | LegacyBidirectionalIterator to `value_type`
`const_iterator` | LegacyBidirectionalIterator to const value_type
`reverse_iterator` | std::reverse_iterator<iterator>
`const_reverse_iterator` | std::reverse_iterator<const_iterator>
`node_type` (since C++17) | a specialization of node handle representing a container node
`insert_return_type` (since C++17) | type describing the result of inserting a `node_type`, a specialization of template<class Iter, class NodeType> struct /*unspecified*/ { Iter position; bool inserted; NodeType node; }; instantiated with template arguments `iterator` and `node_type`.

### Member classes

value_compare | compares objects of type `value_type` (class)

### Member functions

(constructor) | constructs the `map` (public member function)
(destructor) | destructs the `map` (public member function)
operator= | assigns values to the container (public member function)
get_allocator | returns the associated allocator (public member function)
##### Element access
at | access specified element with bounds checking (public member function)
operator[] | access or insert specified element (public member function)
##### Iterators
begin cbegin (C++11) | returns an iterator to the beginning (public member function)
end cend (C++11) | returns an iterator to the end (public member function)
rbegin crbegin (C++11) | returns a reverse iterator to the beginning (public member function)
rend crend (C++11) | returns a reverse iterator to the end (public member function)
##### Capacity
empty | checks whether the container is empty (public member function)
size | returns the number of elements (public member function)
max_size | returns the maximum possible number of elements (public member function)
##### Modifiers
clear | clears the contents (public member function)
insert | inserts elements or nodes (since C++17) (public member function)
insert_range (C++23) | inserts a range of elements (public member function)
insert_or_assign (C++17) | inserts an element or assigns to the current element if the key already exists (public member function)
emplace (C++11) | constructs element in-place (public member function)
emplace_hint (C++11) | constructs elements in-place using a hint (public member function)
try_emplace (C++17) | inserts in-place if the key does not exist, does nothing if the key exists (public member function)
erase | erases elements (public member function)
swap | swaps the contents (public member function)
extract (C++17) | extracts nodes from the container (public member function)
merge (C++17) | splices nodes from another container (public member function)
##### Lookup
count | returns the number of elements matching specific key (public member function)
find | finds element with specific key (public member function)
contains (C++20) | checks if the container contains element with specific key (public member function)
equal_range | returns range of elements matching a specific key (public member function)
lower_bound | returns an iterator to the first element not less than the given key (public member function)
upper_bound | returns an iterator to the first element greater than the given key (public member function)
##### Observers
key_comp | returns the function that compares keys (public member function)
value_comp | returns the function that compares keys in objects of type value_type (public member function)

### Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20) | lexicographically compares the values in the map (function template)
std::swap(std::map) | specializes the std::swap algorithm (function template)
erase_if(std::map) (C++20) | erases all elements satisfying specific criteria (function template)

### Deduction guides | (since C++17)

### Notes

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | 202202L | (C++23) | Ranges construction and insertion for containers

### Example

Run this code

    #include <iostream>
    #include <map>
    #include <string>
    #include <string_view>

    void print_map(std::string_view comment, const std::map<std::string, int>& m)
    {
        std::cout << comment;
        // iterate using C++17 facilities
        for (const auto& [key, value] : m)
            std::cout << '[' << key << "] = " << value << "; ";

    // C++11 alternative:
    //  for (const auto& n : m)
    //      std::cout << n.first << " = " << n.second << "; ";
    //
    // C++98 alternative
    //  for (std::map<std::string, int>::const_iterator it = m.begin(); it != m.end(); it++)
    //      std::cout << it->first << " = " << it->second << "; ";

        std::cout << '\n';
    }

    int main()
    {
        // Create a map of three (string, int) pairs
        std::map<std::string, int> m{{"CPU", 10}, {"GPU", 15}, {"RAM", 20}};

        print_map("1) Initial map: ", m);

        m["CPU"] = 25; // update an existing value
        m["SSD"] = 30; // insert a new value
        print_map("2) Updated map: ", m);

        // using operator[] with non-existent key always performs an insert
        std::cout << "3) m[UPS] = " << m["UPS"] << '\n';
        print_map("4) Updated map: ", m);

        m.erase("GPU");
        print_map("5) After erase: ", m);

        std::erase_if(m, [](const auto& pair){ return pair.second > 25; });
        print_map("6) After erase: ", m);
        std::cout << "7) m.size() = " << m.size() << '\n';

        m.clear();
        std::cout << std::boolalpha << "8) Map is empty: " << m.empty() << '\n';
    }

Output:

    1) Initial map: [CPU] = 10; [GPU] = 15; [RAM] = 20;
    2) Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30;
    3) m[UPS] = 0
    4) Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30; [UPS] = 0;
    5) After erase: [CPU] = 25; [RAM] = 20; [SSD] = 30; [UPS] = 0;
    6) After erase: [CPU] = 25; [RAM] = 20; [UPS] = 0;
    7) m.size() = 3
    8) Map is empty: true

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 230 | C++98 | `Key` was not required to be CopyConstructible (a key of type `Key` might not be able to be constructed) | `Key` is also required to be CopyConstructible
LWG 464 | C++98 | accessing a const `map` by key was inconvenient | `at` function provided

### See also

unordered_map (C++11) | collection of key-value pairs, hashed by keys, keys are unique (class template)

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