C++ Unordered_multimap::hash_function() Function

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The C++ function std::unordered_multimap::hash_function() is used to calculate the hash function object used by the unordered_multimap container.

A hash function object is an instance of a class that has the functionality to generate a unique hash value for a given element. We can only get the hash value of the key of the container.

The hash function is a unary function that takes an object of type key_type as argument and returns a unique value of type size_t based on it.

Syntax

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

hasher hash_function() const;

Parameters

This function does not accepts any parameter.

Return value

This function returns a unique identifier or hash value of the specified argument.

Example 1

In the following example, we are demonstrating the usage of the hash_function().

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap <string, string> umm;
   auto fun = umm.hash_function();
   cout << "Hash function for a = " << fun("a") << endl;
   cout << "Hash function for A = " << fun("A") << endl;
   return 0;
}

Output

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

Hash function for a = 4993892634952068459
Hash function for A = 6919333181322027406

Example 2

Consider the following example, where we are going to use the hash_function() to get the hash value of the specified string value.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap <string, int> umm;
   auto fun = umm.hash_function();
   cout << "Hash value of tutorialspoint = " << fun("tutorialspoint") << endl;
   cout << "Hash value of hyderabad = " << fun("hyderabad") << endl;
   return 0;
}

Output

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

Hash value of tutorialspoint = 1837101086513568625
Hash value of hyderabad = 13759299188341863370

Example 3

Let's look at the following example, where we are going to create theb unordered_multimap and calculate the hash value of each key.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap <string, int> umm = {{"Aman", 1}, {"Vivek", 2}, {"Aman", 1}, {"Vivek", 2}, {"Rahul", 3}};
   auto fun = umm.hash_function();
   for(auto it = umm.begin(); it!=umm.end(); ++it){
      cout << "Hash value of "<<it->first<<" "<<fun(it->first) << endl;  
   }
   return 0;
}

Output

Following is the output of the above code −

Hash value of Vivek 13786444838311805924
Hash value of Vivek 13786444838311805924
Hash value of Rahul 3776999528719996023
Hash value of Aman 17071648282880668303
Hash value of Aman 17071648282880668303

Example 4

Following is the example, in which we are going to use the unordered_multimap that stores the duplicate key and finding the hash value of the specified key using hash_function().

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap <string, int> umm = {{"Aman", 1}, {"Vivek", 2}, {"Aman", 3}, {"Vivek", 4}};
   //syntax of hash function;
   unordered_multimap <string, int> :: hasher fun = umm.hash_function();
   cout<<"Hash value of Aman : "<<fun("Aman")<<endl;
   
   for(auto it = umm.begin(); it!=umm.end(); ++it){
      cout << "{"<<it->first<<" "<<it->second<<"}" << endl;  
   }
   return 0;
}

Output

Output of the above code is as follows −

Hash value of Aman : 17071648282880668303
{Vivek 4}
{Vivek 2}
{Aman 3}
{Aman 1}