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std::unordered_set emplace() method

// Non const version only
template< class... Args >
std::pair<iterator, bool> emplace( Args&&... args );

Inserts a new element into the container constructed in-place with the given args if there is no element with the key in the container.

Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations.

The constructor of the new element is called with exactly the same arguments as supplied to emplace, forwarded via std::forward<Args>(args).... The element may be constructed even if there already is an element with the key in the container, in which case the newly constructed element will be destroyed immediately..

Invalidation

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().

Parameters

  • args - arguments to forward to the constructor of the element

Return value

Returns a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and a bool denoting whether the insertion took place (true if insertion happened, false if it did not).

Complexity

Average case, constant - O(1).
Worst case, linear in size of the container - O(size()).

Exceptions

If an exception is thrown by any operation, this function has no effect (strong exception guarantee).

Example

important

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std::unordered_set emplace() method

// Non const version only
template< class... Args >
std::pair<iterator, bool> emplace( Args&&... args );

Inserts a new element into the container constructed in-place with the given args if there is no element with the key in the container.

Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations.

The constructor of the new element is called with exactly the same arguments as supplied to emplace, forwarded via std::forward<Args>(args).... The element may be constructed even if there already is an element with the key in the container, in which case the newly constructed element will be destroyed immediately..

Invalidation

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().

Parameters

  • args - arguments to forward to the constructor of the element

Return value

Returns a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and a bool denoting whether the insertion took place (true if insertion happened, false if it did not).

Complexity

Average case, constant - O(1).
Worst case, linear in size of the container - O(size()).

Exceptions

If an exception is thrown by any operation, this function has no effect (strong exception guarantee).

Example

important

This section requires improvement. You can help by editing this doc page.

This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.