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      std::multiset

      From cppreference.com
      <cpp‎ |container
       
       
       
      std::multiset
      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<set>
      template<

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

      >class multiset;
      		 (1)
      namespace pmr{

         template<
             class Key,
             class Compare=std::less<Key>
         >using multiset= std::multiset<Key, Compare,std::pmr::polymorphic_allocator<Key>>;

      }
      		 (2)(since C++17)

      std::multiset is an associative container that contains a sorted set of objects of type Key. Unlike set, multiple keys with equivalent values are allowed. Sorting is done using the key comparison function Compare. Search, insertion, and removal operations have logarithmic complexity.

      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).

      The order of the elements that compare equivalent is the order of insertion and does not change.

      		(since C++11)

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

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

      However,std::multiset 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]
      value_typeKey[edit]
      size_type Unsigned integer type (usuallystd::size_t)[edit]
      difference_type Signed integer type (usuallystd::ptrdiff_t)[edit]
      key_compareCompare[edit]
      value_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]
      iterator ConstantLegacyBidirectionalIterator 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 functions

      constructs themultiset
      (public member function)[edit]
      destructs themultiset
      (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 twomultisets
      (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

      The member typesiterator andconst_iterator may be aliases to the same type. This means defining a pair of function overloads using the two types as parameter types may violate theOne Definition Rule. Sinceiterator is convertible toconst_iterator, a single function with aconst_iterator as parameter type will work instead.

      Feature-test macroValueStdFeature
      __cpp_lib_containers_ranges202202L(C++23)Ranges construction and insertion for containers
      __cpp_lib_constexpr_set202502L(C++26)constexprstd::multiset

      [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 103C++98iterator allows modification of keysiterator made constant
      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 unique keys, sorted by keys
      (class template)[edit]
      collection of keys, hashed by keys
      (class template)[edit]
      adapts a container to provide a collection of keys, sorted by keys
      (class template)[edit]
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