C++ unordered_multimap::max_bucket_count() Function

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The C++ std::unordered_multimap::max_bucket_count() function is used to return the maximum number of buckets that the unordered_multimap container can hold, based on its current memory allocation. each bucket in an unordered_multimap is a container that holds element with same hash value.

A bucket is a slot in the container's internal hash table to which elements are assigned based on the hash value of their key.

Syntax

Following is the syntax of the std::unordered_multimap:: max_bucket_count() function.

size_type max_bucket_count() const;

Parameters

This function does not accepts any parameter.

Return value

This function returns an unsigned integer that represent the maximum number of buckets.

Example 1

Let's look at the following example, where we are going to use the max_bucket_count() function on empty multimap and observing the output.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> um;
   cout << "max_bucket_count of unordered_multimap = " << um.max_bucket_count() << endl;
   return 0;
}

Output

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

max_bucket_count of unordered_multimap = 576460752303423487

Example 2

Consider the another scenario, where we are going to get the maximum number of buckets in the current multimap using the max_bucket_count.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> umm={{'A', 1}, {'B', 2}, {'A', 3}, {'B', 4}, {'C', 5}};
   cout << "max_bucket_count of unordered_multimap = " << umm.max_bucket_count() << endl;
   return 0;
}

Output

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

max_bucket_count of unordered_multimap = 576460752303423487

Example 3

In the following example, we are creating an unordered_multimap that stores the negative key value and then using the max_bucket_count to get the maximum number of bucket, as follows −

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<int, char> umm={{-1, 'a'}, {-2, 'b'}, {-3, 'c'}, {-2, 'd'}, {-3, 'e'}};
   cout << "max_bucket_count of unordered_multimap = " << umm.max_bucket_count() << endl;
   return 0;
}

Output

Following is the output of the above code −

max_bucket_count of unordered_multimap = 576460752303423487

Example 4

In the following example, we are going to get the maximum bucket count before and after the insertion of elements into the empty multimap.

#include <iostream>
#include <unordered_map>
using namespace std;
  
int main() {
   unordered_multimap<char, int> ummap;  
   cout << "Size is : " << ummap.size() << endl;
   cout << "Max bucket count is : " << ummap.max_bucket_count() << endl;
	
   // insert elements
   ummap.insert({ 'a', 10 });
   ummap.insert({ 'b', 12 });
   ummap.insert({ 'c', 13 });
   ummap.insert({ 'b', 12 });
   ummap.insert({ 'd', 15 });
   ummap.insert({ 'e', 20 });
  
   cout << "Size is : " << ummap.size() << endl;
   cout << "Max bucket count is : " << ummap.max_bucket_count() << endl;
   return 0;
}

Output

Output of the above code is as follows −

Size is : 0
Max bucket count is : 576460752303423487
Size is : 6
Max bucket count is : 576460752303423487