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unordered_set hash_function() in C++ STL

Last Updated : 05 Jun, 2023
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The unordered_set::hash_function() is a built-in function in C++ STL which is used to get hash function. This hash function is a unary function which takes asingle argument only and returns a unique value of type size_t based on it. 

Syntax:

unordered_set_name.hash_function()

Parameter: The function accepts no parameter. 

Return Value: The function returns the hash function. Below programs illustrate the unordered_set::hash_function() function:

 Program 1: 

CPP 
// CPP program to illustrate the 
// unordered_set::hash() function 

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

using namespace std; 

int main() 
{ 

    unordered_set<string> sampleSet = { "geeks1", "for", "geeks2" }; 

    // use of hash_function 
    unordered_set<string>::hasher fn = sampleSet.hash_function(); 

    cout << fn("geeks") << endl; 

    for (auto it = sampleSet.begin(); it != sampleSet.end(); it++) { 
        cout << *it << " "; 
    } 
    cout << endl; 

    return 0; 
}
Output:
15276750567035005396
geeks2 geeks1 for

Program 2: 

CPP 
// CPP program to illustrate the 
// unordered_set::hash() function 

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

using namespace std; 

int main() 
{ 

    unordered_set<string> sampleSet; 

    // use of hash_function 
    unordered_set<string>::hasher fn = sampleSet.hash_function(); 

    cout << fn("geeksforgeeks") << endl; 

    return 0; 
}
Output:
5146686323530302118

Time complexity: O(1)


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