unordered_set reserve() function in C++ STL

The unordered_set::reserve() method is a builtin function in C++ STL which is used to request capacity change of unordered_set. It sets the number of buckets in the container 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 the bucket_count, then the function has no effect on it.

Syntax:

unordered_set_name.reserve(size_type n)

Parameter: The function accepts a single mandatory parameter n which sets the number of buckets in the container (bucket_count) to the most appropriate to contain at least n elements.

Return Value: This function doesn’t returns anything.

Below programs illustrate the unordered_set::reserve() function:

Program 1:

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// C++ program to illustrate
// the unordered_set.reserve()
#include <iostream>
#include <string>
#include <unordered_set>
  
using namespace std;
  
int main()
{
    // Declaration of unordered_set
    unordered_set<string> us;
  
    us.reserve(3);
  
    us.insert("geeks");
    us.insert("for");
    us.insert("geeks");
    us.insert("users");
    us.insert("geeksforgeeks");
  
    for (auto it = us.begin(); it != us.end(); it++) {
        cout << *it << " ";
    }
  
    return 0;
}

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Output:


geeksforgeeks users geeks for





Program 2: 











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// C++ program to illustrate 


// the unordered_set.reserve() 


#include <iostream> 


#include <string> 


#include <unordered_set> 


  


using namespace std; 


  


int main() 


{ 


    // Declaration of unordered_set 


    unordered_set<string> us; 


  


    us.reserve(0); 


  


    us.insert("geeks"); 


    us.insert("for"); 


    us.insert("geeks"); 


  


    for (auto it = us.begin(); it != us.end(); it++) { 


        cout << *it << " "; 


    } 


  


    return 0; 


} 



















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Output:


for geeks






          
          
          
          

            

    

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