C++ unordered_multimap::reserve() Function

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

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

Syntax

Following is the syntax for std::unordered_multimap::reserve() function.

void reserve(size_type n);

Parameters

• n − It indicates the capacity of the container.

Return value

This function does not return anything.

Example 1

In the following example, let's see the usage of unordered_multimap::reserve() function.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> umm;
   cout << "Initial bucket count = " << umm.bucket_count() << endl;
   umm.reserve(5);
   cout << "Bucket count after reserve = "
      << umm.bucket_count() << endl;
   return 0;
}

Output

On executing the above code, we get the following output, which shows the initial bucket count and reserve count −

Initial bucket count = 1
Bucket count after reserve = 5

Example 2

Consider the following example, where we are going to use the reserve() function to make the bucket count to store at least 5 element.

#include <iostream>
#include <unordered_map>
using namespace std;
int main () {
   unordered_multimap<string, string> umMap;
   cout << "Bucket Count: " << umMap.bucket_count() << endl;
   umMap.reserve(5);
   cout << "Bucket Count after reserve(): " << umMap.bucket_count() << endl;
  
   umMap.insert({ {"Fname", "Tutorix"}, {"Fname", "tutorials"},{"Lname", "Point"}, {"Country","India"}, {"Locaton","Hyderabad"}});

   for (auto& it: umMap)
      cout << it.first << "->" << it.second << endl;
   return 0;
}

Output

On executing the above code, we get the bucket count in both the initial and reserve containers, as well as the element of the container.

Bucket Count: 1
Bucket Count after reserve(): 5
Country->India
Lname->Point
Locaton->Hyderabad
Fname->tutorials
Fname->Tutorix

Example 3

Let's look at the following example, where we are going to display the bucket with its elements before and after the use of the reserve() function.

#include <iostream>
#include <unordered_map>
using namespace std;
int main () {
   unordered_multimap<string, string> umMap={{"Hyderabad", "India"}, {"Delhi", "India"}, {"Bangalore", "India"}, {"Hyderabad", "Telngana"}};
   cout<<"Unordered_multimap contains "<<umMap.bucket_count()<<" buckets:";
   for(unsigned int i = 0; i < umMap.bucket_count(); i++) {
      cout<<"\nThe bucket "<<i<<" contains: ";   
      for(auto it = umMap.begin(i); it != umMap.end(i); ++it) {
         cout<<it->first<<":"<<it->second<<" ";
	  } 
   }  

   cout<<"\nCapacity is changed using reserve function.\n";
   umMap.reserve(5);
   
   cout<<"Unordered_multimap contains "<<umMap.bucket_count()<<" buckets:";
   for(unsigned int i = 0; i < umMap.bucket_count(); i++) {
      cout<<"\nThe bucket "<<i<<" contains: ";   
      for(auto it = umMap.begin(i); it != umMap.end(i); ++it) {
         cout<<it->first<<":"<<it->second<<" ";
      } 
   }
   return 0;
}

Output

On executing the above code, we get the number of buckets in both initial and reserve conditions and their elements −

Unordered_multimap contains 5 buckets:
The bucket 0 contains: 
The bucket 1 contains: Bangalore:India 
The bucket 2 contains: 
The bucket 3 contains: Delhi:India 
The bucket 4 contains: Hyderabad:Telngana Hyderabad:India 
Capacity is changed using reserve function.
Unordered_multimap contains 5 buckets:
The bucket 0 contains: 
The bucket 1 contains: Bangalore:India 
The bucket 2 contains: 
The bucket 3 contains: Delhi:India 
The bucket 4 contains: Hyderabad:Telngana Hyderabad:India