std::unordered_set::insert(3)

NAME
std::unordered_set::insert - std::unordered_set::insert

Synopsis
   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 > (5) (since C++11)
   void insert( InputIt first, InputIt last );
   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 > (9) (since C++23)
   std::pair<iterator, bool> insert( K&& obj );
   template< class K > (10) (since C++23)
   iterator insert( const_iterator hint, K&& obj );
   Inserts element(s) into the container, if the container doesn't already contain an
   element with an equivalent key.
   1,2) Inserts value.
   3,4) Inserts value, using hint 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 (pending
   LWG2844).
   6) Inserts elements from initializer list ilist. If multiple elements in the range
   have keys that compare equivalent, it is unspecified which element is inserted
   (pending LWG2844).
   7) If nh is an empty node handle, does nothing. Otherwise, inserts the element owned
   by nh into the container , if the container doesn't already contain an element with
   a key equivalent to nh.key(). The behavior is undefined if nh is not empty and
   get_allocator() != nh.get_allocator().
   8) If nh is an empty node handle, does nothing and returns the end iterator.
   Otherwise, inserts the element owned by nh into the container, if the container
   doesn't already contain an element with a key equivalent to nh.key(), and returns
   the iterator pointing to the element with key equivalent to nh.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 if nh is not
   empty and get_allocator() != nh.get_allocator().
   9) If *this already contains an element which transparently compares equivalent to
   obj, does nothing. Otherwise, constructs an object u of value_type with
   std::forward<K>(obj) and then inserts u into *this. If equal_range(u) !=
   hash_function()(obj) || contains(u) is true, the behavior is undefined. The
   value_type must be EmplaceConstructible into unordered_set from
   std::forward<K>(obj). This overload participates in overload resolution only if
   Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type.
   This assumes that such Hash is callable with both K and Key type, and that the
   KeyEqual is transparent, which, together, allows calling this function without
   constructing an instance of Key.
   10) If *this already contains an element which transparently compares equivalent to
   obj, does nothing.
   Otherwise, constructs an object u of value_type with std::forward<K>(obj) and then
   inserts u into *this. Template:hint is used as a non-binding suggestion to where the
   search should start. If equal_range(u) != hash_function()(obj) || contains(u) is
   true, the behavior is undefined. The value_type must be EmplaceConstructible into
   unordered_set from std::forward<K>(obj). This overload participates in overload
   resolution only if:
     * std::is_convertible_v<K&&, const_iterator> and std::is_convertible_v<K&&,
       iterator> are both false, and
     * Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a
       type. This assumes that such Hash is callable with both K and Key type, and that
       the KeyEqual is transparent,
   which, together, allows calling this function without constructing an instance of
   Key.
   If after the operation the new number of elements is greater than old
   max_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)

Parameters
   hint - iterator, used as a suggestion as to where to insert the content
   value - element value to insert
   first, last - range of elements to insert
   ilist - initializer list to insert the values from
   nh - a compatible node handle
   obj - a value of any type that can be transparently compared with a key

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.

Return value
   1,2) A pair consisting of an iterator to the inserted element (or to the element
   that prevented the insertion) and a bool value set to true 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 of insert_return_type with the members initialized as follows:
     * If nh is empty, inserted is false, position is end(), and node is empty.
     * Otherwise if the insertion took place, inserted is true, position points to the
       inserted element, and node is empty.
     * If the insertion failed, inserted is false, node has the previous value of nh,
       and position points to an element with a key equivalent to nh.key().
   8) End iterator if nh was empty, iterator pointing to the inserted element if
   insertion took place, and iterator pointing to an element with a key equivalent to
   nh.key() if it failed.
   9) A pair consisting of an iterator to the inserted element (or to the element that
   prevented the insertion) and a bool value set to true if and only if the insertion
   took place.
   10) An iterator to the inserted element, or to the element that prevented the
   insertion.

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

Complexity
   1-4) Average case: O(1), worst case O(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 case O(size()).

Notes
   The hinted insert (3,4) does not return a boolean in order to be
   signature-compatible with positional insert on sequential containers, such as
   std::vector::insert. This makes it possible to create generic inserters such as
   std::inserter. One way to check success of a hinted insert is to compare size()
   before and after.
                Feature-test macro Value Std Feature
                        Heterogeneous
                        overloads for the
                        remaining member
   __cpp_lib_associative_heterogeneous_insertion 202311L (C++26) functions in ordered
                        and unordered
                        associative
                        containers. (9,10)

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: true
 3) 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 }

See also
   emplace constructs element in-place
                (public member function)
   emplace_hint constructs elements in-place using a hint
                (public member function)
   inserter creates a std::insert_iterator of type inferred from the argument
                (function template)

Category:
     * Todo with reason