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Showing content from https://cplusplus.com/reference/unordered_set/unordered_set/reserve/ below:

public member function

<unordered_set>

std::unordered_set::reserve 
void reserve ( size_type n );

Request a capacity change

Sets the number of buckets in the container (bucket_count) to the most appropriate to contain at least n elements.

If n is greater than the current bucket_count multiplied by the max_load_factor, the container's bucket_count is increased and a rehash is forced.

If n is lower than that, the function may have no effect.



Parameters
n
The number of elements requested as minimum capacity.
Member type size_type is an unsigned integral type.

Return value none

Example 
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// unordered_set::reserve
#include <iostream>
#include <string>
#include <unordered_set>

int main ()
{
  std::unordered_set<std::string> myset;

  myset.reserve(5);

  myset.insert("office");
  myset.insert("house");
  myset.insert("gym");
  myset.insert("parking");
  myset.insert("highway");

  std::cout << "myset contains:";
  for (const std::string& x: myset) std::cout << " " << x;
  std::cout << std::endl;

  return 0;
}

Possible output: 
myset contains: highway house office gym parking

By calling reserve with the size we expected for the unordered_set container we avoided the multiple rehashes that the increases in container size could have produced and optimized the size of the hash table.



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.
If no actual rehash happens, no changes.

See also
unordered_set::rehash
Set number of buckets (public member function)
unordered_set::bucket_count
Return number of buckets (public member function)
unordered_set::max_load_factor
Get or set maximum load factor (public member function)
unordered_set::size
Return container size (public member function)

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