unordered_multiset reserve() in C++ STL

The reserve() function of unordered_multiset 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.

Syntax:

void reserve( size_type n );

where size_type is an unsigned integral type.

Parameters: This method accepts a mandatory parameter n which is the number of elements requested as minimum capacity.

Returns: It does not returns any value.

Below are the programs to illustrate reserve() method:

Example 1:

#include <iostream>
#include <unordered_set>

using namespace std;

int main()
{
    unordered_multiset<int> j;

    // container invoked
    // it reverse the values
    j.reserve(5);

    // set the values of the container
    j.insert(5);
    j.insert(6);
    j.insert(7);

    cout << "The values in unordered_multiset :";
    for (const int& x : j)
        cout << " " << x;

    return 0;
}

Output:

The values in unordered_multiset : 7 6 5

Example 2:

#include <iostream>
#include <unordered_set>

using namespace std;

int main()
{
    unordered_multiset<string> j;

    // container invoked
    // it reverse the values
    j.reserve(5);

    // set the values of the container
    j.insert("Geeks");
    j.insert("forGeeks");
    j.insert("GeeksforGeeks");

    cout << "The values in unordered_multiset :";
    for (const string& x : j)
        cout << " " << x;

    return 0;
}

Output:

The values in unordered_multiset : GeeksforGeeks forGeeks Geeks