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std::hash

Creepy Cormorant answered on April 16, 2022 Popularity 8/10 Helpfulness 2/10

answer std::hash

std::hash

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#include <iostream>
#include <iomanip>
#include <functional>
#include <string>
#include <unordered_set>
 
struct S {
    std::string first_name;
    std::string last_name;
};
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()(S const& 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()(S const& 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) << ") = " << 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) << ") = "
              << MyHash{}(obj) << " (using MyHash)\n" << std::setw(31) << "or "
              << std::hash<S>{}(obj) << " (using injected std::hash<S> 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& s: names)
        std::cout << std::quoted(s.first_name) << ' ' << std::quoted(s.last_name) << '\n';
}

Popularity 8/10 Helpfulness 2/10 Language cpp

Source: en.cppreference.com

Tags: c++

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Contributed on Apr 16 2022

Creepy Cormorant

0 Answers Avg Quality 2/10

std::hash

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