unordered_map rehash in C++ STL

The std::unordered_map::rehash() is a built in function C++ STL which which sets the number of buckets in container to n or more.
Syntax

void rehash( size_type s );

If s is greater than the current buckets into the container then rehashed is done. The new bucket count can be greater than or equal to n.
If s is less than current bucket count then there may or may not be any effect of function. It totally depends upon compiler

Parameters : It takes New number of buckets into the container.

Return type : Its return type is none.

Example-1:

// C++ code to illustrate the method 
// unordered_map rehash 
#include <bits/stdc++.h> 
  
using namespace std; 
  
int main() 
{ 
    unordered_map<char, int> sample; 
  
    // Map initialization 
    sample = { { 'a', 2 }, { 'b', 4 }, { 'c', 6 } }; 
  
    cout << " Size of container : " << sample.size() << endl; 
    cout << " Initial bucket count : " << sample.bucket_count() << endl; 
  
    // changing rehash 
  
    sample.rehash(30); 
    cout << " Size of container : " << sample.size() << endl; 
    cout << " Now bucket count is :  " << sample.bucket_count() << endl; 
    return 0; 
} 

Output:

Size of container : 3
 Initial bucket count : 5
 Size of container : 3
 Now bucket count is :  31

Example-2:

// C++ code to illustrate the method 
// unordered_map rehash 
#include <bits/stdc++.h> 
  
using namespace std; 
  
int main() 
{ 
    unordered_map<int, int> sample; 
  
    // Map initialization 
    sample = { { 2, 2 }, { 3, 4 }, { 4, 6 }, { 5, 8 } }; 
  
    cout << " Size of container : " << sample.size() << endl; 
    cout << " Initial bucket count : " << sample.bucket_count() << endl; 
  
    // changing rehash 
  
    sample.rehash(20); 
    cout << " Size of container : " << sample.size() << endl; 
    cout << " Now bucket count is :  " << sample.bucket_count() << endl; 
    return 0; 
} 

Output:

Size of container : 4
 Initial bucket count : 7
 Size of container : 4
 Now bucket count is :  23

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