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

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< cpp‎ | container‎ | unordered set

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)

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. The element is inserted as close as possible to `hint`. The behavior is undefined if `nh` is not empty and get_allocator() != nh.get_allocator().

If rehashing occurs due to the insertion, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is greater than max_load_factor()*bucket_count(). 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
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.

### Return value

1-2) Returns a pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool denoting whether the insertion took place.
3-4) Returns an iterator to the inserted element, or to the element that prevented the insertion.
5-6) (none)
7) Returns an `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.

### Exceptions

1-4) If an exception is thrown by any operation, the insertion has no effect.

| This section is incomplete Reason: cases 5-8

### 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. Worse case: `O(N*size()+N)`
7-8) 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.

### See also

emplace | constructs element in-place (public member function)
emplace_hint | constructs elements in-place using a hint (public member function)

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