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C++ Unordered Set Hash Function

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.

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

hasher hash_function() const;

This function does not accepts any parameter.

This function returns the hash function.

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;
}

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

This contains: 16508604031426688195
That contains: 12586652871572997328

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;
}

Following is the output of the above code −

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

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 of the above code is as follows −

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

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