std::map(3)

NAME
std::map - std::map

Synopsis
   Defined in header <map>
   template<
       class Key,
       class T, (1)
       class Compare = std::less<Key>,
       class Allocator = std::allocator<std::pair<const Key, T>>
   > class map;
   namespace pmr {
       template<
           class Key,
           class T,
           class Compare = std::less<Key> (2) (since C++17)
       > using map = std::map<Key, T, Compare,
    std::pmr::polymorphic_allocator<std::pair<const Key, T>>>;
   }
   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.
   Iterators of std::map iterate in ascending order of keys, where ascending is defined
   by the comparison that was used for construction. That is, given
     * m, a std::map
     * it_l and it_r, dereferenceable iterators to m, with it_l < it_r.
   m.value_comp()(*it_l, *it_r) == true (least to greatest if using the default
   comparison).
   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&
                        Allocator::pointer (until C++11)
   pointer std::allocator_traits<Allocator>::pointer (since C++11)
                        Allocator::const_pointer (until
                        C++11)
   const_pointer 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
                        type describing the result of inserting a node_type, a
                        specialization of
                        template<class Iter, class NodeType>
                        struct /*unspecified*/
   insert_return_type (since {
   C++17) 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 returns an iterator to the beginning
   cbegin (public member function)
   (C++11)
   end returns an iterator to the end
   cend (public member function)
   (C++11)
   rbegin returns a reverse iterator to the beginning
   crbegin (public member function)
   (C++11)
   rend returns a reverse iterator to the end
   crend (public member function)
   (C++11)

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)
                    inserts elements
   insert or nodes
                    (since C++17)
                    (public member function)
   insert_range inserts a range of elements
   (C++23) (public member function)
   insert_or_assign inserts an element or assigns to the current element if the key
   (C++17) already exists
                    (public member function)
   emplace constructs element in-place
   (C++11) (public member function)
   emplace_hint constructs elements in-place using a hint
   (C++11) (public member function)
   try_emplace inserts in-place if the key does not exist, does nothing if the key
   (C++17) exists
                    (public member function)
   erase erases elements
                    (public member function)
   swap swaps the contents
                    (public member function)
   extract extracts nodes from the container
   (C++17) (public member function)
   merge splices nodes from another container
   (C++17) (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 checks if the container contains element with specific key
   (C++20) (public member function)
   equal_range returns range of elements matching a specific key
                    (public member function)
                    returns an iterator to the first element not less than the given
   lower_bound 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)
                    returns the function that compares keys in objects of type
   value_comp value_type
                    (public member function)

Non-member functions
   operator==
   operator!=
   operator<
   operator<=
   operator>
   operator>= lexicographically compares the values of two maps
   operator<=> (function template)
   (removed in C++20)
   (removed in C++20)
   (removed in C++20)
   (removed in C++20)
   (removed in C++20)
   (C++20)
   std::swap(std::map) specializes the std::swap algorithm
                       (function template)
   erase_if(std::map) erases all elements satisfying specific criteria
   (C++20) (function template)
     Deduction guides (since C++17)

Notes
       Feature-test macro Value Std Feature
   __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
                      Key was not required to be
   LWG 230 C++98 CopyConstructible Key is also required to
                      (a key of type Key might not be able to be CopyConstructible
                      be constructed)
   LWG 464 C++98 accessing a const map by key was at function provided
                      inconvenient

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

Category:
     * Todo with reason