C++ Unordered_set::hash_function() Function

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The C++ std::unordered_set::hash_function() is used to get the hash function object of the allocated elements, which is calculated by the hash function object used by the unordered_set 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. It is a unary function that accepts key_type objuects as argument and returns a unique value of type size_t based on it.

Syntax

Following is the syntax of std::unordered_set::hash_function().

hasher hash_function() const;

Parameters

This function does not accepts any parameter.

Return Value

This function returns the hash function.

Example 1

Consider the following example, where we are going to demonstrate the usage of std::unordered_set::hash_function() function.

#include <iostream>
#include <string>
#include <unordered_set>

typedef std::unordered_set<std::string> stringset;

int main () {
   stringset uSet;

   stringset::hasher fn = uSet.hash_function();

   std::cout << "This contains: " << fn ("This") << std::endl;
   std::cout << "That contains: " << fn ("That") << std::endl;

   return 0;
}

Output

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

This contains: 16508604031426688195
That contains: 12586652871572997328

Example 2

Let's look at the following example, where we are going to calculate the hash value of each element of the unordered_set.

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

int main(void) {
   unordered_set <string> uSet = {"Aman", "Vivek", "Rahul"};
   auto fun = uSet.hash_function();
   for(auto it = uSet.begin(); it!=uSet.end(); ++it){
      cout << "Hash value of "<<*it<<" "<<fun(*it) << endl;  
   }
   return 0;
}

Output

Following is the output of the above code −

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

Example 3

In the following example, we are going to calculate the hash value of each element of the unordered_set of type char.

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

int main(void) {
   unordered_set <char> uSet = {'a', 'b', 'c'};
   auto fun = uSet.hash_function();
   for(auto it = uSet.begin(); it!=uSet.end(); ++it){
      cout << "Hash value of "<<*it<<" "<<fun(*it) << endl;  
   }
   return 0;
}

Output

Output of the above code is as follows −

Hash value of c 99
Hash value of b 98
Hash value of a 97