"The C++ Library doesen't provide a hash for this type." - Using own class in std::unordered_map

Question

I am trying to code Conway's "Game of Life". While getting closer to my goal I got stuck with a compiler error:

C2338: The C++ Library doesen't provide a hash for this type.

At first I used the SFML class sf::Vector2D. When it failed to work for me I wrote a class of my own, hoping I could implement the missing hashCode method.

My question is:
Is it possible to uses my own class with its own hashCode method for std::unordered_map? I need to use a class, that can hold two numbers. (I also tried std::tuple, struct and stuff).

Here is one sheet of my code:

#include "GameMechanics.h"



GameMechanics::GameMechanics(Elements * elements):elements(elements)
{
    this->refreshTime = 1000000;    //ms
    this->clock.restart();
}


GameMechanics::~GameMechanics()
{
}

bool GameMechanics::isRunning()
{
    return this->running;
}

void GameMechanics::setRunning(bool running)
{
    this->running = running;
}

void GameMechanics::loop()
{

    unsigned passedTime = clock.getElapsedTime().asMicroseconds();  //check passed time since the clock got restarted
    this->timeHeap  +=  passedTime; //add passed time to the timeheap
    this->clock.restart();
    //only refresh every "refreshTime" seconds
    if (timeHeap >= this->refreshTime) {    
        std::cout << "Calculated new generation!" << std::endl;
        this->timeHeap -= this->refreshTime;
        this->calculateNextGeneration();
    }
}

void GameMechanics::calculateNextGeneration()
{
    std::list<sf::Vector2i> oldGeneration = this->elements->getElements();  //  population in the moment

    sf::Vector2u elements = this->elements->getElementCount();

    std::unordered_map<MyVector2D, int> counter;     //here is the problem. Thats the line that makes some trouble


    for (std::list<sf::Vector2i>::iterator it = oldGeneration.begin(); it != oldGeneration.end(); it++) {
        sf::Vector2i position = *it;

        for (int i = -1; i < 2; i++) 
        {
            for (int j = -1; j < 2; j++) 
            {
                if (position.x + i >= 0 && position.x + i <= this->elements->getElementCount().x &&
                    position.y + j >= 0 && position.y + j <= this->elements->getElementCount().y) 
                {
                    if (counter.find(MyVector2D(position.x + i, position.y + j)) != counter.end()) 
                    {
                        counter.at(MyVector2D(position.x + i, position.y + j))++;
                    }
                    else //if there is no such element, create a new entry
                    {
                        counter.insert({ MyVector2D(position.x + i, position.y + j),1 });
                    }
                }
            }
        }
    }


    //create new generation
    this->brithNewGeneration(&counter);
}

void GameMechanics::brithNewGeneration(std::unordered_map<MyVector2D,int>* counter)
{
    //this methode does work

    std::list<sf::Vector2i> newGeneration;
//  for (std::unordered_map<MyVector2D, int>::iterator it = counter->begin(); it != counter->end(); it++) 
    {
        //if life vell with < 2 neighbours, it dies
        //life cell with 2 or 3 neighbours will continue living

        //life cell with >4 cells will die

        //dead cell with 3 neighbours will start living
    }
}

Unrelated: unless you are inserting elements in the middle of the sequence all the time, std::list is almost never a good container choice. — Matteo Italia
– Matteo Italia, Commented May 14, 2017 at 13:03

Quentin · Accepted Answer · 2017-05-14 12:56:38Z

6

The custom hashing function required for std::unordered_map (and std::unordered_set) is not a member function of the stored type. You need to specialize the std::hash template:

namespace std {
    template<>
    struct hash<YourType> {
        using argument_type = YourType;
        using result_type = std::size_t;

        result_type operator()(argument_type const &obj) const {
            // Compute and return the hash value for `obj`.
        }
    };
}

Your case is precisely the reason why it's done this way: you can specialize std::hash for sf::Vector2D if you wish, no need to implement your own class.

answered May 14, 2017 at 12:56

Quentin

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1 Comment

Matteo Italia Over a year ago

Related: stackoverflow.com/a/38140932/214671; in this specific case, my MAKE_HASHABLE macro would reduce the whole solution to MAKE_HASHABLE(sf::Vector2i, t.x, t.y).

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