std::hash

#include <cstddef>
#include <functional>
#include <iomanip>
#include <iostream>
#include <string>
#include <unordered_set>
 
struct S
{
 std::string first_name;
 std::string last_name;
 bool operator==(const S&) const = default; // since C++20
};
 
// Before C++20.
// bool operator==(const S& lhs, const S& rhs)
// {
// return lhs.first_name == rhs.first_name && lhs.last_name == rhs.last_name;
// }
 
// Custom hash can be a standalone function object.
struct MyHash
{
 std::size_t operator()(const S& s) const noexcept
 {
 std::size_t h1 = std::hash<std::string >{}(s.first_name);
 std::size_t h2 = std::hash<std::string >{}(s.last_name);
 return h1 ^ (h2 << 1); // or use boost::hash_combine
 }
};
 
// Custom specialization of std::hash can be injected in namespace std.
template<>
struct std::hash<S>
{
 std::size_t operator()(const S& s) const noexcept
 {
 std::size_t h1 = std::hash<std::string >{}(s.first_name);
 std::size_t h2 = std::hash<std::string >{}(s.last_name);
 return h1 ^ (h2 << 1); // or use boost::hash_combine
 }
};
 
int main()
{
 std::string str = "Meet the new boss...";
 std::size_t str_hash = std::hash<std::string >{}(str);
 std::cout << "hash(" << std::quoted (str) << ") =\t" << str_hash << '\n';
 
 S obj = {"Hubert", "Farnsworth"};
 // Using the standalone function object.
 std::cout << "hash(" << std::quoted (obj.first_name) << ", "
 << std::quoted (obj.last_name) << ") =\t"
 << MyHash{}(obj) << " (using MyHash) or\n\t\t\t\t"
 << std::hash<S>{}(obj) << " (using injected specialization)\n";
 
 // Custom hash makes it possible to use custom types in unordered containers.
 // The example will use the injected std::hash<S> specialization above,
 // to use MyHash instead, pass it as a second template argument.
 std::unordered_set <S> names = {obj, {"Bender", "Rodriguez"}, {"Turanga", "Leela"}};
 for (auto const& s: names)
 std::cout << std::quoted (s.first_name) << ' '
 << std::quoted (s.last_name) << '\n';
}

hash("Meet the new boss...") = 10656026664466977650
hash("Hubert", "Farnsworth") = 12922914235676820612 (using MyHash) or
 12922914235676820612 (using injected specialization)
"Bender" "Rodriguez"
"Turanga" "Leela"
"Hubert" "Farnsworth"