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      std::multimap

      From cppreference.com
      <cpp‎ |container
       
       
       
      std::multimap
      Member functions
      Non-member functions
      (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
      Deduction guides(C++17)
       
      Defined in header<map>
      template<

         class Key,
         class T,
         class Compare=std::less<Key>,
         class Allocator=std::allocator<std::pair<const Key, T>>

      >class multimap;
      		 (1)
      namespace pmr{

         template<
             class Key,
             class T,
             class Compare=std::less<Key>
         >using multimap= std::multimap<Key, T, Compare,
             std::pmr::polymorphic_allocator<std::pair<const Key, T>>>;

      }
      		 (2)(since C++17)

      std::multimap is an associative container that contains a sorted list of key-value pairs, while permitting multiple entries with the same key. Sorting is done according to the comparison functionCompare, applied to the keys. Search, insertion, and removal operations have logarithmic complexity.

      Iterators ofstd::multimap iterate in non-descending order of keys, where non-descending is defined by the comparison that was used for construction. That is, given

      • m, astd::multimap
      • it_l andit_r, dereferenceable iterators tom, withit_l< it_r.

      m.value_comp()(*it_r,*it_l)==false (least to greatest if using the default comparison).

      The order of the key-value pairs whose keys compare equivalent is the order of insertion and does not change.

      		(since C++11)

      Everywhere the standard library uses theCompare requirements, equivalence is determined by using the equivalence relation as described onCompare. In imprecise terms, two objectsa andb are considered equivalent if neither compares less than the other:!comp(a, b)&&!comp(b, a).

      std::multimap meets the requirements ofContainer,AllocatorAwareContainer,AssociativeContainer andReversibleContainer.

      All member functions ofstd::multimap areconstexpr: it is possible to create and usestd::multimap objects in the evaluation of a constant expression.

      However,std::multimap objects generally cannot beconstexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.

      		(since C++26)

      Contents

      [edit]Template parameters

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

      [edit]Member types

      Type Definition
      key_typeKey[edit]
      mapped_typeT[edit]
      value_typestd::pair<const Key, T>[edit]
      size_type Unsigned integer type (usuallystd::size_t)[edit]
      difference_type Signed integer type (usuallystd::ptrdiff_t)[edit]
      key_compareCompare[edit]
      allocator_typeAllocator[edit]
      referencevalue_type&[edit]
      const_referenceconst value_type&[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]
      iteratorLegacyBidirectionalIterator andConstexprIterator(since C++26) tovalue_type[edit]
      const_iteratorLegacyBidirectionalIterator andConstexprIterator(since C++26) toconst value_type[edit]
      reverse_iteratorstd::reverse_iterator<iterator>[edit]
      const_reverse_iteratorstd::reverse_iterator<const_iterator>[edit]
      node_type(since C++17) a specialization ofnode handle representing a container node[edit]

      [edit]Member classes

      compares objects of typevalue_type
      (class)[edit]

      [edit]Member functions

      constructs themultimap
      (public member function)[edit]
      destructs themultimap
      (public member function)[edit]
      assigns values to the container
      (public member function)[edit]
      returns the associated allocator
      (public member function)[edit]
      Iterators
      returns an iterator to the beginning
      (public member function)[edit]
      (C++11)
      		 returns an iterator to the end
      (public member function)[edit]
      returns a reverse iterator to the beginning
      (public member function)[edit]
      (C++11)
      		 returns a reverse iterator to the end
      (public member function)[edit]
      Capacity
      checks whether the container is empty
      (public member function)[edit]
      returns the number of elements
      (public member function)[edit]
      returns the maximum possible number of elements
      (public member function)[edit]
      Modifiers
      clears the contents
      (public member function)[edit]
      inserts elementsor nodes(since C++17)
      (public member function)[edit]
      inserts a range of elements
      (public member function)[edit]
      (C++11)
      		 constructs element in-place
      (public member function)[edit]
      constructs elements in-place using a hint
      (public member function)[edit]
      erases elements
      (public member function)[edit]
      swaps the contents
      (public member function)[edit]
      (C++17)
      		 extracts nodes from the container
      (public member function)[edit]
      (C++17)
      		 splices nodes from another container
      (public member function)[edit]
      Lookup
      returns the number of elements matching specific key
      (public member function)[edit]
      finds element with specific key
      (public member function)[edit]
      (C++20)
      		 checks if the container contains element with specific key
      (public member function)[edit]
      returns range of elements matching a specific key
      (public member function)[edit]
      returns an iterator to the first elementnot less than the given key
      (public member function)[edit]
      returns an iterator to the first elementgreater than the given key
      (public member function)[edit]
      Observers
      returns the function that compares keys
      (public member function)[edit]
      returns the function that compares keys in objects of typevalue_type
      (public member function)[edit]

      [edit]Non-member functions

      (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 of twomultimaps
      (function template)[edit]
      specializes thestd::swap algorithm
      (function template)[edit]
      erases all elements satisfying specific criteria
      (function template)[edit]

      Deduction guides

      		(since C++17)

      [edit]Notes

      Feature-test macroValueStdFeature
      __cpp_lib_containers_ranges202202L(C++23)Ranges construction and insertion for containers
      __cpp_lib_constexpr_map202502L(C++26)constexprstd::multimap

      [edit]Example

      This section is incomplete
      Reason: no example

      [edit]Defect reports

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

      DRApplied toBehavior as publishedCorrect behavior
      LWG 230C++98Key was not required to beCopyConstructible
      (a key of typeKey might not be able to be constructed)      		Key is also required to
      beCopyConstructible

      [edit]See also

      collection of key-value pairs, sorted by keys, keys are unique
      (class template)[edit]
      collection of key-value pairs, hashed by keys
      (class template)[edit]
      adapts two containers to provide a collection of key-value pairs, sorted by keys
      (class template)[edit]
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