C++ Unordered_map Library - reserve() Function

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Description

The C++ function std::unordered_map::reserve() sets the number of buckets in the container to the most appropriate to contain at least n elements.

If n is greater than the current bucket_count() * max_load_factor() then the container's bucket count is increased and a rehash is forced and if n is lower than that, the function may have no effect.

Declaration

Following is the declaration for std::unordered_map::reserve() function form std::unordered_map header.

C++11

void reserve(size_type n);

Parameters

n − New capacity of the container.

Return value

None

Time complexity

Linear i.e. O(n) in average case.

Quadratic i.e. O(n²) in worst case.

Example

The following example shows the usage of std::unordered_map::reserve() function.

#include <iostream>
#include <unordered_map>

using namespace std;

int main(void) {
   unordered_map<char, int> um;

   cout << "Initial bucket count = " << um.bucket_count() << endl;

   um.reserve(5);

   cout << "Bucket count after reserve = "
       << um.bucket_count() << endl;

   return 0;
}

Let us compile and run the above program, this will produce the following result −

Initial bucket count = 11
Bucket count after reserve = 5