This is a small demo showing how to properly use std::unordered_set in C++

unordered_set_demo.cpp

/*
 * std::unordered_set unlike a std::set is implementation of chaining hash table.
 * Therefore, it requires additional implementation to get it to work.
 * This demo tries to show the requirements for such an implementation.
*/

#include <iostream>
#include <algorithm>
#include <unordered_set>
#include <functional>

using namespace std;

class Person 
{
public:
    int age, height;

    Person(int a, int h): age(a), height(h) {
    }

    bool operator<(const Person& a_person) const {
        return this->age < a_person.age;
    }

    bool operator==(const Person& a_person) const {
        return this->age == a_person.age && this->height == a_person.height;
    }
};

namespace std
{
    template <>
    struct hash<Person>
    {
        size_t operator()(const Person& p) const {
            size_t ret_val = hash<int>()(p.age) + hash<int>()(p.height);
            cout << "hash = " << ret_val << endl;
            return ret_val;
        }
    };
}

int main()
{
    unordered_set<Person> p_set;

    p_set.insert(Person(10,4));
    p_set.insert(Person(9,3));
    p_set.insert(Person(14,5));

    for_each(begin(p_set), end(p_set), [](const Person& p) { cout << p.age << " and " << p.height << endl;});
}