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

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
}
}

Answer 1

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.