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      std::map

      From cppreference.com
      <cpp‎ |container
       
       
       
      std::map
      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 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 functionCompare. Search, removal, and insertion operations have logarithmic complexity. Maps are usually implemented asRed–black trees.

      Iterators ofstd::map iterate in ascending order of keys, where ascending is defined by the comparison that was used for construction. That is, given

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

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

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

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

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

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

      		(since C++26)

      Contents

      Template parameters

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

      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]
      insert_return_type(since C++17) type describing the result of inserting anode_type, a specialization of

      template<class Iter,class NodeType>
      struct/*unspecified*/
      {
          Iter     position;
         bool     inserted;
          NodeType node;
      };
      instantiated with template argumentsiterator andnode_type.[edit]

      Member classes

      compares objects of typevalue_type
      (class)[edit]

      Member functions

      constructs themap
      (public member function)[edit]
      destructs themap
      (public member function)[edit]
      assigns values to the container
      (public member function)[edit]
      returns the associated allocator
      (public member function)[edit]
      Element access
      access specified element with bounds checking
      (public member function)[edit]
      access or insert specified element
      (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]
      inserts an element or assigns to the current element if the key already exists
      (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]
      inserts in-place if the key does not exist, does nothing if the key exists
      (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]

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

      Deduction guides

      		(since C++17)

      Notes

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

      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 facilitiesfor(constauto&[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 insertstd::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,[](constauto& 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] = 04) 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() = 38) Map is empty: true

      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
      LWG 464C++98accessing a constmap by key was inconvenientat function provided

      See also

      collection of key-value pairs, sorted by keys
      (class template)[edit]
      collection of key-value pairs, hashed by keys, keys are unique
      (class template)[edit]
      (C++23)
      		 adapts two containers to provide a collection of key-value pairs, sorted by unique keys
      (class template)[edit]
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