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      std::unordered_set<Key,Hash,KeyEqual,Allocator>::insert

      From cppreference.com
      <cpp‎ |container‎ |unordered set
       
       
       
      std::unordered_set
      Member types
      Member functions
      Non-member functions
      Deduction guides(C++17)
       
      std::pair<iterator,bool> insert(const value_type& value);
      		 (1)(since C++11)
      std::pair<iterator,bool> insert( value_type&& value);
      		 (2)(since C++11)
      iterator insert( const_iterator hint,const value_type& value);
      		 (3)(since C++11)
      iterator insert( const_iterator hint, value_type&& value);
      		 (4)(since C++11)
      template<class InputIt>
      void insert( InputIt first, InputIt last);
      		 (5)(since C++11)
      void insert(std::initializer_list<value_type> ilist);
      		 (6)(since C++11)
      insert_return_type insert( node_type&& nh);
      		 (7)(since C++17)
      iterator insert( const_iterator hint, node_type&& nh);
      		 (8)(since C++17)
      template<class K>
      std::pair<iterator,bool> insert( K&& obj);
      		 (9)(since C++23)
      template<class K>
      iterator insert( const_iterator hint, K&& obj);
      		 (10)(since C++23)

      Inserts element(s) into the container, if the container does not already contain an element with an equivalent key.

      1,2) Insertsvalue.
      3,4) Insertsvalue, usinghint as a non-binding suggestion to where the search should start.
      5) Inserts elements from range[first, last). If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pendingLWG2844).
      6) Inserts elements from initializer listilist. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pendingLWG2844).
      7) Ifnh is an emptynode handle, does nothing. Otherwise, inserts the element owned bynh into the container , if the container doesn't already contain an element with a key equivalent tonh.key(). The behavior is undefined ifnh is not empty andget_allocator()!= nh.get_allocator().
      8) Ifnh is an emptynode handle, does nothing and returns the end iterator. Otherwise, inserts the element owned bynh into the container, if the container doesn't already contain an element with a key equivalent tonh.key(), and returns the iterator pointing to the element with key equivalent tonh.key()(regardless of whether the insert succeeded or failed). If the insertion succeeds,nh is moved from, otherwise it retains ownership of the element.hint is used as a non-binding suggestion to where the search should start. The behavior is undefined ifnh is not empty andget_allocator()!= nh.get_allocator().
      9) If*this already contains an element which transparently comparesequivalent toobj, does nothing. Otherwise, constructs an objectu ofvalue_type withstd::forward<K>(obj) and then insertsu into*this. Ifequal_range(u)!= hash_function()(obj)|| contains(u) istrue, the behavior is undefined. Thevalue_type must beEmplaceConstructible intounordered_set fromstd::forward<K>(obj). This overload participates in overload resolution only ifHash andKeyEqual are bothtransparent. This assumes that suchHash is callable with bothK andKey type, and that theKeyEqual is transparent, which, together, allows calling this function without constructing an instance ofKey.
      10) If*this already contains an element which transparently comparesequivalent toobj, does nothing.

      Otherwise, constructs an objectu ofvalue_type withstd::forward<K>(obj) and then insertsu into*this.Template:hint is used as a non-binding suggestion to where the search should start. Ifequal_range(u)!= hash_function()(obj)|| contains(u) istrue, the behavior is undefined.Thevalue_type must beEmplaceConstructible intounordered_set fromstd::forward<K>(obj). This overload participates in overload resolution only if:

      • std::is_convertible_v<K&&, const_iterator> andstd::is_convertible_v<K&&, iterator> are bothfalse, and
      • Hash::is_transparent andKeyEqual::is_transparent are valid and each denotes a type. This assumes that suchHash is callable with bothK andKey type, and that theKeyEqual is transparent,
      which, together, allows calling this function without constructing an instance ofKey.

      If after the operation the new number of elements is greater than oldmax_load_factor() * bucket_count() a rehashing takes place.
      If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated.If the insertion is successful, pointers and references to the element obtained while it is held in the node handle are invalidated, and pointers and references obtained to that element before it was extracted become valid.(since C++17)

      Contents

      [edit]Parameters

      hint - iterator, used as a suggestion as to where to insert the content
      value - element value to insert
      first, last - the pair of iterators defining the sourcerange of elements to insert
      ilist - initializer list to insert the values from
      nh - a compatiblenode handle
      obj - a value of any type that can be transparently compared with a key
      Type requirements
      -
      InputIt must meet the requirements ofLegacyInputIterator.

      [edit]Return value

      1,2) A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and abool value set totrue if and only if the insertion took place.
      3,4) An iterator to the inserted element, or to the element that prevented the insertion.
      5,6) (none)
      7) An object ofinsert_return_type with the members initialized as follows:
      • Ifnh is empty,inserted isfalse,position isend(), andnode is empty.
      • Otherwise if the insertion took place,inserted istrue,position points to the inserted element, andnode is empty.
      • If the insertion failed,inserted isfalse,node has the previous value ofnh, andposition points to an element with a key equivalent tonh.key().
      8) End iterator ifnh was empty, iterator pointing to the inserted element if insertion took place, and iterator pointing to an element with a key equivalent tonh.key() if it failed.
      9) A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and abool value set totrue if and only if the insertion took place.
      10) An iterator to the inserted element, or to the element that prevented the insertion.

      [edit]Exceptions

      1-4) If an exception is thrown by any operation, the insertion has no effect.
      This section is incomplete
      Reason: cases 5-10

      [edit]Complexity

      1-4) Average case:O(1), worst caseO(size()).
      5,6) Average case:O(N), where N is the number of elements to insert. Worst case:O(N * size() + N).
      7-10) Average case:O(1), worst caseO(size()).

      [edit]Notes

      The hinted insert((3,4),(8) and(10))does not return a boolean in order to be signature-compatible with positional insert on sequential containers, such asstd::vector::insert. This makes it possible to create generic inserters such asstd::inserter. One way to check success of a hinted insert is to comparesize() before and after.

      Feature-test macroValueStdFeature
      __cpp_lib_associative_heterogeneous_insertion202311L(C++26)Heterogeneous overloads for the remaining member functions inordered andunordered associativecontainers.(9,10)

      [edit]Example

      Run this code
      #include <array>#include <iostream>#include <unordered_set> std::ostream& operator<<(std::ostream& os,std::unordered_set<int>const& s){for(os<<'['<< s.size()<<"] { ";int i: s)        os<< i<<' ';return os<<"}\n";} int main(){std::unordered_set<int> nums{2,3,4}; std::cout<<"1) Initially: "<< nums<<std::boolalpha;auto p= nums.insert(1);// insert element, overload (1)std::cout<<"2) '1' was inserted: "<< p.second<<'\n';std::cout<<"3) After insertion: "<< nums;     nums.insert(p.first,0);// insert with hint, overload (3)std::cout<<"4) After insertion: "<< nums; std::array<int,4> a={10,11,12,13};    nums.insert(a.begin(), a.end());// insert range, overload (5)std::cout<<"5) After insertion: "<< nums;     nums.insert({20,21,22,23});// insert initializer_list, (6)std::cout<<"6) After insertion: "<< nums; std::unordered_set<int> other_nums={42,43};auto node= other_nums.extract(other_nums.find(42));    nums.insert(std::move(node));// insert node, overload (7)std::cout<<"7) After insertion: "<< nums;     node= other_nums.extract(other_nums.find(43));    nums.insert(nums.begin(), std::move(node));// insert node with hint, (8)std::cout<<"8) After insertion: "<< nums;}

      Possible output:

      1) Initially: [3] { 4 3 2 }2) '1' was inserted: true3) After insertion: [4] { 1 2 3 4 }4) After insertion: [5] { 0 1 2 3 4 }5) After insertion: [9] { 13 12 11 10 4 3 2 1 0 }6) After insertion: [13] { 23 22 13 12 11 10 21 4 20 3 2 1 0 }7) After insertion: [14] { 42 23 22 13 12 11 10 21 4 20 3 2 1 0 }8) After insertion: [15] { 43 42 23 22 13 12 11 10 21 4 20 3 2 1 0 }

      [edit]See also

      constructs element in-place
      (public member function)[edit]
      constructs elements in-place using a hint
      (public member function)[edit]
      creates astd::insert_iterator of type inferred from the argument
      (function template)[edit]
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