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C++ Unordered_map::hash_function() Function
The C++ std::unordered_map::hash_function() function is used to calculates the hash function object used by the unordered_map 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.
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_map::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_map <string, string> um; auto fun = um.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_map <string, int> um; auto fun = um.hash_function(); cout << "Hash value of tutorialspoint = " << fun("tutorialspoint") << endl; cout << "Hash value of hyderabad = " << fun("hyderabad") << endl; return 0; }
Output
Following is the output of the above code −
Hash value of tutorialspoint = 1837101086513568625 Hash value of hyderabad = 13759299188341863370
Example 3
Let's look at the following example, where we going to create an unordered_map and calculate the hash value of each key.
#include <iostream> #include <unordered_map> using namespace std; int main(void) { unordered_map <string, int> um = {{"Aman", 1}, {"Vivek", 2}, {"Rahul", 3}}; auto fun = um.hash_function(); for(auto it = um.begin(); it!=um.end(); ++it){ cout << "Hash value of "<<it->first<<" "<<fun(it->first) << endl; } return 0; }
Output
Output of the above code is as follows −
Hash value of Rahul 3776999528719996023 Hash value of Vivek 13786444838311805924 Hash value of Aman 17071648282880668303