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

    From cppreference.com
     
    C++
     
    Containers library
     
    std::unordered_set
    Member types
    Member functions
    Non-member functions
    (until C++20)
    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)
    See More

    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 - the pair of iterators defining the source 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
    __cpp_lib_associative_heterogeneous_insertion 202311L (C++26) Heterogeneous overloads for the remaining member 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

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