std::hash class in C++ STL

Difficulty Level : Hard
Last Updated : 23 Jul, 2019

The hash class is default constructible, which means that one can construct this object without any arguments or initialization values. It is used to get the hash value of the argument that is being passed to it. If the argument doesn’t change, the value doesn’t change either.

Syntax:

template <class T> struct hash;

Syntax to create object:

hash<class template> object-name

Member functions: This Hash class only has one member function:

operator(): It returns hashed value for given argument.

Below is the implementation of the hash class:

Example:

#include <bitset> 
  
// functional header 
// for hash<class template> class 
#include <functional> 
  
#include <iostream> 
#include <string> 
#include <vector> 
  
using namespace std; 
  
// To demonstrate String Hashing 
void stringHashing() 
{ 
  
    // Get the string 
    // to get its hash value 
    string hashing1 = "Geeks"; 
  
    // Instatiation of Object 
    hash<string> mystdhash; 
  
    // Using operator() to get hash value 
    cout << "String hash values: "
         << mystdhash(hashing1) 
         << endl; 
} 
  
// To demonstrate BITSET Hashing 
void bitsetHashing() 
{ 
  
    // Get the BITSET 
    // to get its hash value 
    bitset<5> h_bitset("10101"); 
  
    // Instatiation of Object 
    hash<bitset<5> > hash_bitset; 
  
    // Using operator() to get hash value 
    cout << "\nBitset 10101 hash value: "
         << hash_bitset(h_bitset) << endl; 
} 
  
// To demonstrate Vector<bool> Hashing 
void vectorHashing() 
{ 
  
    // Get the Vector<bool> 
    // to get its hash value 
    vector<bool> 
        h_vecbool{ true, false, 
                   true, false }; 
  
    // Instatiation of Object 
    hash<vector<bool> > hash_vector_bool; 
  
    // Using operator() to get hash value 
    cout << "\nVector<bool> hash value: "
         << hash_vector_bool(h_vecbool) 
         << endl; 
} 
  
// To demonstrate Char Hashing 
void charHashing() 
{ 
  
    // Get the char 
    // to get its hash value 
    char ch = 'a'; 
  
    // Instatiation of Object 
    hash<char> hash_char; 
  
    // Using operator() to get hash value 
    cout << "\nChar hash values: "
         << hash_char(ch) 
         << endl; 
} 
  
// Driver Code 
int main() 
{ 
  
    stringHashing(); 
    bitsetHashing(); 
    vectorHashing(); 
    charHashing(); 
} 

Output:

String hash values: 4457761756728957899

Bitset 10101 hash value: 17123654466142159564

Vector hash value: 16935082123893034051

Char hash values: 97

Reference: http://www.cplusplus.com/reference/functional/hash/

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