The std::hash Template Class in C++

1. Use std::hash to Generate Hash for std::string Objects
2. Use std::hash to Generate Hash for std::bitset Objects
3. Use std::hash to Generate Hash for std::vector<bool> Objects

This article will introduce the std::hash template class from STL in C++.

Use std::hash to Generate Hash for std::string Objects

The std::hash template class is provided under the STL <functional> header. It creates a hash function object. std::hash satisfies the requirements of the DefaultConstructible type, and it only requires to have template argument supplied.

Multiple default specializations of this template class are provided in C++ standard library, and the full list can be seen here. Once the hash function object is created with the given template argument, it can be utilized to generate specific hash values using operator() that accepts a single argument and returns the size_t value.

In the next example, we use string specialization and generate some hash values for arbitrary strings.

#include <functional>
#include <iomanip>
#include <iostream>

using std::cout;
using std::endl;
using std::setw;
using std::string;

int main() {
  string str1("arbitrary string");
  std::vector<string> vec2 = {"true", "false", "false",
                              "true", "false", "true"};

  std::hash<string> str_hash;

  cout << "hash(str1) - " << str_hash(str1) << endl;

  cout << "hash(vec2 elements) - ";
  for (const auto &item : vec2) {
    cout << str_hash(item) << ", ";
  }
  cout << endl;

  return EXIT_SUCCESS;
}

Output:

hash(str1) - 3484993726433737991
hash(vec2 elements) - 1325321672193711394, 3658472883277130625, 3658472883277130625, 1325321672193711394, 3658472883277130625, 1325321672193711394,

Use std::hash to Generate Hash for std::bitset Objects

Another specialization of std::hash provided in the STL is for std::bitset arguments. Remember that std::bitset is the class that represents a fixed number of bits as a sequence, and it provides multiple member functions for easy bit manipulation.

Generally, the hash functions used by std::hash specializations are implementation-dependent, and one should not use these objects as a universal solution to hashing problem. Also, these hash functions are only required to produce the same output for the same input within a single execution of the program.

#include <bitset>
#include <functional>
#include <iomanip>
#include <iostream>

using std::cout;
using std::endl;
using std::setw;
using std::string;

int main() {
  std::bitset<8> b1("00111001");

  std::hash<std::bitset<8>> bitset_hash;

  cout << "hash(bitset<8>) - " << bitset_hash(b1) << endl;

  return EXIT_SUCCESS;
}

Output:

hash(bitset<8>) - 6623666755989985924

Use std::hash to Generate Hash for std::vector<bool> Objects

We can also use std::hash specialization for a vector of boolean values, as shown in the following code snippet. Note that std::hash can also be specialized for user-defined classes, and some additional specializations are available through Boost library (details of which are listed here.

#include <functional>
#include <iomanip>
#include <iostream>

using std::cout;
using std::endl;
using std::setw;
using std::string;

int main() {
  std::vector<bool> vec1 = {true, false, false, true, false, true};

  std::hash<std::vector<bool> > vec_str_hash;

  cout << "hash(vector<bool>) - " << vec_str_hash(vec1) << endl;

  return EXIT_SUCCESS;
}

Output:

hash(vector<bool>) - 12868445110721718657