public member function
<unordered_map>

std::unordered_map::emplace_hint

template <class... Args>
iterator emplace_hint ( const_iterator position, Args&&... args );
Construct and insert element with hint
Inserts a new element in the unordered_map if its key is unique. This new element is constructed in place using args as the arguments for the element's constructor. position points to a location in the container suggested as a hint on where to start the search for its insertion point (the container may or may not use this suggestion to optimize the insertion operation).

The insertion only takes place if no element in the container has a key equivalent to the one being emplaced (keys in an unordered_map are unique).

If inserted, this effectively increases the container size by one.

A similar member function exists, insert, which either copies or moves an existing object into the container, and may also take a position hint.

Parameters

position
Position suggested as a hint for the insertion operation. This value may be used by the container to optimize the operation.
Member type const_iterator is a forward iterator type.
args
Arguments used to construct a new object of the mapped type for the inserted element.
Because the mapped type is only part of the value type, the standard specification is -at least- ambiguous here. If implemented by your library, it may have been interpreted as either:
  • The first argument in the list is treated as the key and the remaining arguments are used to construct an object of the mapped type in place.
  • The arguments are used to construct a new object of the value type and passed to the pair constructor.

Return value

An iterator to the element in the container with the key equivalent to the newly inserted element. This points to the newly inserted element if it was indeed inserted, or to the existing element with the equivalent key if one already existed (it is not replaced).

Member type iterator is a forward iterator.

The storage for the new element is allocated using allocator_traits<allocator_type>::construct(), which may throw exceptions on failure (for the default allocator, bad_alloc is thrown if the allocation request does not succeed).

Example

No example yet. See unordered_map::emplace's example.

Complexity

Average case: constant.
Worst case: linear in container size.
May trigger a rehash (not included).

Iterator validity

On most cases, all iterators in the container remain valid after the insertion. The only exception being when the growth of the container forces a rehash. In this case, all iterators in the container are invalidated.

A rehash is forced if the new container size after the insertion operation would increase above its capacity threshold (calculated as the container's bucket_count multiplied by its max_load_factor).

References remain valid in all cases, even after a rehash.

See also