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C++ Unordered Set Reserve

The C++ unordered_set::reserve() function is used to change the capacity of the bucket. It sets the number of buckets in the container (bucket_count) to the most appropriate to contain at least n elements without exceeding maximum load factor and rehashes the container.

A rehash is forced when n is greater than the current bucket_count multiplied by the max_load_factor, and increase in the container bucket count, causes a rehash.

Following is the syntax of std::unordered_set::reserve() function.

void reserve ( size_type n );

• n − It inidicates the minimum number of buckets.

This function does not returns anything.

Let's look at the following example, where we are going to demonstrate the usage of unordered_set::reserve() function.

#include <iostream>
#include <string>
#include <unordered_set>
using namespace std;

int main () {
   unordered_set<string> uSet;
   
   uSet.reserve(3);
   uSet.insert("android");
   uSet.insert("java");
   uSet.insert("html");
   
   cout << "uSet contains:";
   
   for (const string& x: uSet) cout << " " << x;
   cout << endl;
   
   cout<<"After reserve bucket count is: "<< uSet.bucket_count();
   return 0;
}

If we run the above code it will generate the following output −

uSet contains: html java android
After reserve bucket count is: 3

In the following example, we are going to count the number of buckets before and after the use of the unordered_set::reserve() function.

#include <iostream>
#include <string>
#include <unordered_set>
using namespace std;

int main () {
   unordered_set<string> uSet = {"android", "java", "Html", "CSS"};
   cout<<"Initial bucket count is: "<< uSet.bucket_count() << endl;
   
   uSet.reserve(3);
   
   cout << "uSet contains:";
   for (const string& x: uSet)
      cout << " " << x;
   cout << endl;
   cout<<"After reserve bucket count is: "<< uSet.bucket_count();
   return 0;
}

Following is the output of the above code −

Initial bucket count is: 13
uSet contains: android java Html CSS
After reserve bucket count is: 5

Consider the following example, where we are going to display the buckets and their elements before and after the use of the unordered_set::reserve() function.

#include <iostream>
#include <unordered_set>
using namespace std;

int main () {
   unordered_set<string> uSet={"Hyderabad", "Delhi", "Bangalore"};
   cout<<"unordered_set contains "<<uSet.bucket_count()<<" buckets:";
   for(unsigned int i = 0; i < uSet.bucket_count(); i++) {
      cout<<"\nThe bucket "<<i<<" contains: ";   
      for(auto it = uSet.begin(i); it != uSet.end(i); ++it) {
         cout<<*it<<" ";
	  } 
   }  

   cout<<"\n***Capacity is changed using reserve function***\n";
   uSet.reserve(5);
   
   cout<<"unordered_set contains "<<uSet.bucket_count()<<" buckets:";
   for(unsigned int i = 0; i < uSet.bucket_count(); i++) {
      cout<<"\nThe bucket "<<i<<" contains: ";   
      for(auto it = uSet.begin(i); it != uSet.end(i); ++it) {
         cout<<*it<<" ";
      } 
   }
   return 0;
}

Output of the above code is as follows −

unordered_set contains 13 buckets:
The bucket 0 contains: Bangalore 
The bucket 1 contains: 
The bucket 2 contains: Hyderabad 
The bucket 3 contains: Delhi 
The bucket 4 contains: 
The bucket 5 contains: 
The bucket 6 contains: 
The bucket 7 contains: 
The bucket 8 contains: 
The bucket 9 contains: 
The bucket 10 contains: 
The bucket 11 contains: 
The bucket 12 contains: 
***Capacity is changed using reserve function***
unordered_set contains 5 buckets:
The bucket 0 contains: 
The bucket 1 contains: Bangalore 
The bucket 2 contains: 
The bucket 3 contains: Delhi 
The bucket 4 contains: Hyderabad 

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