std::unordered_map::reserve(3)

NAME
std::unordered_map::reserve - std::unordered_map::reserve

Synopsis
   void reserve( size_type count ); (since C++11)
   Sets the number of buckets to the number needed to accommodate at least count
   elements without exceeding maximum load factor and rehashes the container, i.e. puts
   the elements into appropriate buckets considering that total number of buckets has
   changed. Effectively calls rehash(std::ceil(count / max_load_factor())).

Parameters
   count - new capacity of the container

Return value
   (none)

Complexity
   Average case linear in the size of the container, worst case quadratic.

See also
          reserves at least the specified number of buckets and regenerates the hash
   rehash table
          (public member function)