How can I use an unordered_set with a custom struct?

Question

I want to use an unordered_set with a custom struct . In my case, the custom struct represents a 2D point in an euclidean plane. I know that one should define a hash function and comparator operator and I have done so as you can see in my code below:

struct Point {
    int X;
    int Y;

    Point() : X(0), Y(0) {};
    Point(const int& x, const int& y) : X(x), Y(y) {};
    Point(const IPoint& other){
        X = other.X;
        Y = other.Y;
    };

    Point& operator=(const Point& other) {
        X = other.X;
        Y = other.Y;
        return *this;
    };

    bool operator==(const Point& other) {
        if (X == other.X && Y == other.Y)
            return true;
        return false;
    };

    bool operator<(const Point& other) {
        if (X < other.X )
            return true;
        else if (X == other.X && Y == other.Y)
            return true;

        return false;
    };

    size_t operator()(const Point& pointToHash) const {
        size_t hash = pointToHash.X + 10 * pointToHash.Y;
        return hash;
    };
};

However, I'm getting the error below, if I define the set as follows:

unordered_set<Point> mySet;

Error C2280 'std::hash<_Kty>::hash(const std::hash<_Kty> &)': attempting to reference a deleted function

What am I missing?

Answer 1

The second template parameter to std::unordered_set is the type to use for hashing. and will default to std::hash<Point> in your case, which doesn't exist. So you can use std::unordered_set<Point,Point> if the hasher is the same type.

Alternatively if you do not want to specify the hasher, define a specialization of std::hash for Point and either get rid of the member function and implement the hashing in the body of your specialization's operator() , or call the member function from the std::hash specialization.

#include <unordered_set>

struct Point {
    int X;
    int Y;

    Point() : X(0), Y(0) {};
    Point(const int& x, const int& y) : X(x), Y(y) {};
    Point(const Point& other){
        X = other.X;
        Y = other.Y;
    };

    Point& operator=(const Point& other) {
        X = other.X;
        Y = other.Y;
        return *this;
    };

    bool operator==(const Point& other) const {
        if (X == other.X && Y == other.Y)
            return true;
        return false;
    };

    bool operator<(const Point& other) {
        if (X < other.X )
            return true;
        else if (X == other.X && Y == other.Y)
            return true;

        return false;
    };

    // this could be moved in to std::hash<Point>::operator()
    size_t operator()(const Point& pointToHash) const noexcept {
        size_t hash = pointToHash.X + 10 * pointToHash.Y;
        return hash;
    };

};

namespace std {
    template<> struct hash<Point>
    {
        std::size_t operator()(const Point& p) const noexcept
        {
            return p(p);
        }
    };
}


int main()
{
    // no need to specify the hasher if std::hash<Point> exists
    std::unordered_set<Point> p;
    return 0;
}

Demo

Answer 2

While the above solution gets you compiling code, avoid that hash function for points. There's a one dimensional subspace parameterized by b for which all points on the line y = -x/10 + b will have the same hash value. You'd be better off with a 64 bit hash where the top 32 bits are the x coord and the low 32 bits are the y coord (for example). That'd look like

uint64_t hash(Point const & p) const noexcept
{
    return ((uint64_t)p.X)<<32 | (uint64_t)p.Y;
}

Answer 3

I'd like to expand on rmawatson's answer by providing some more tips:

1. For your struct , you neither need to define operator= nor Point(const Point& other) , because you (re-)implemented the default behavior.

2. You can streamline operator== by removing the if clause as follows:

    bool operator==(const Point& other) { return X == other.X && Y == other.Y; };

3. There is a mistake in your operator< : In the else if clause, you return true if both points are equal. This violates the requirement for a strict weak ordering . Therefore, I recommend to use the following code instead:

    bool operator<(const Point& other) { return X < other.X || (X == other.X && Y < other.Y); };

Moreover, since C++11 , you can use lambda expressions instead of defining the hash and comparison functions. This way, you don't need to specify any operators for your struct , if you don't need them otherwise. Putting everything together, your code could be written as follows:

struct Point {
    int X, Y;

    Point() : X(0), Y(0) {};
    Point(const int x, const int y) : X(x), Y(y) {};
};

int main() {
    auto hash = [](const Point& p) { return p.X + 10 * p.Y; };
    auto equal = [](const Point& p1, const Point& p2) { return p1.X == p2.X && p1.Y == p2.Y; };
    std::unordered_set<Point, decltype(hash), decltype(equal)> mySet(8, hash, equal);

    return 0;
}

However, as also explained in CJ13's answer , your hash function might not be the best one. Another way to handcraft a hash function is the following:

auto hash = [](const Point& p) { return std::hash<int>()(p.X) * 31 + std::hash<int>()(p.Y); };

The idea for a more general solution to hashing can be found here .

Code on Ideone