public member function
<unordered_set>

std::unordered_multiset::rehash

void rehash ( size_type n );
Set number of buckets
Sets the number of buckets in the container to n or more.

If n is greater than the current number of buckets in the container (bucket_count), a rehash is forced. The new bucket count can either be equal or greater than n.

If n is lower than the current number of buckets in the container (bucket_count), the function may have no effect on the bucket count and may not force a rehash.

A rehash is the reconstruction of the hash table: All the elements in the container are rearranged according to their hash value into the new set of buckets. This may alter the order of iteration of elements within the container, although the relative order of the elements with equivalent values is preserved.

Rehashes are automatically performed by the container whenever its load factor is going to surpass its max_load_factor in an operation.

Notice that this function expects the number of buckets as argument. A similar function exists, unordered_multiset::reserve, that expects the number of elements in the container as argument.

Parameters

n
The minimum number of buckets for the container hash table.
Member type size_type is an unsigned integral type.

Return value

none

Example

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// unordered_multiset::rehash
#include <iostream>
#include <string>
#include <unordered_set>

int main ()
{
  std::unordered_multiset<std::string> myums;

  myums.rehash(12);

  myums.insert("red");
  myums.insert("red");
  myums.insert("blue");
  myums.insert("green");
  myums.insert("yellow");

  std::cout << "current bucket_count: " << myums.bucket_count() << std::endl;

  return 0;
}


Possible output:
current bucket_count: 13

By calling rehash to reserve a certain minimum amount of buckets in the hash table, we avoid the multiple rehashes that the expansion of the container may cause.

Complexity

In case of rehash,
Average case: linear in container size.
Worst case: quadratic in container size.

Iterator validity

If a rehash happens, all iterators are invalidated, but references and pointers to individual elements remain valid.
The relative order of iteration of equivalent elements is preserved.
If no actual rehash happens, no changes.

See also

  • 9316991614 2018-02-25
  • 191841613 2018-02-25
  • 16411612 2018-02-25
  • 6081981611 2018-02-25
  • 1784561610 2018-02-25
  • 4651609 2018-02-25
  • 9348111608 2018-02-25
  • 2513771607 2018-02-25
  • 6478461606 2018-02-24
  • 4985791605 2018-02-24
  • 5637141604 2018-02-24
  • 282181603 2018-02-24
  • 6217941602 2018-02-24
  • 5076141601 2018-02-24
  • 714281600 2018-02-24
  • 6607141599 2018-02-24
  • 949041598 2018-02-24
  • 6809961597 2018-02-24
  • 671871596 2018-02-24
  • 7107821595 2018-02-24