Implement hash for custom class C++

Question

I have a circle defined like so:

class Circle {
public:
    int x;
    int y;
    int r;

    Circle() : x(0), y(0), r(0) {}

    Circle(int x, int y, int r) {
        this->x = x;
        this->y = y;
        this->r = r;
    }

    double area() {
        return PI * pow(r, 2);
    }
};

I want to be able to add it to a set, based on the hash of its center (x and y)

What's the correct and idiomatic way of doing this in C++ 11?

My question is two-fold

(1) Is there a way I can ask C++ to hash it for me? In Kotlin, there is the notion of a dataclass which automatically hashes the class attributes. I am looking for something similar.

(2) If not, how do I hash it myself, and relatedly - what are operators I need to overload?

Answer 1

I want to be able to add it to a set, based on the hash of its center (x and y)

If you wan't to put your circle in the std::set, you don't need to provide a hash function. You need to provide it only if you'd like to use std::unordered_set or any other unordered associative container (more info in hash).

As it goes for the most idiomatic way of implementation, I would go with something like this (taken from the std::hash example):

#include <cmath>
#include <unordered_set>
#include <set>

class Circle {
public:
    int x;
    int y;
    int r;

    Circle() : x(0), y(0), r(0) {}

    Circle(int x, int y, int r) {
        this->x = x;
        this->y = y;
        this->r = r;
    }

    double area() {
        return 3.1415 * pow(r, 2);
    }
};

bool operator==(const Circle& lhs, const Circle& rhs) {
    return lhs.x == rhs.x && lhs.y == rhs.y;
}

template<> struct std::hash<Circle> {
    std::size_t operator()(Circle const& s) const noexcept {
        std::size_t h1 = std::hash<int>{}(s.x);
        std::size_t h2 = std::hash<int>{}(s.y);
        return h1 ^ (h2 << 1); // or use boost::hash_combine (see Discussion) https://en.cppreference.com/w/Talk:cpp/utility/hash
    }
};

int main() {
    std::unordered_set<Circle> circles;
    return 0;
}

Link to example

EDIT:

(1) Is there a way I can ask C++ to hash it for me? In Kotlin, there is the notion of a dataclass which automatically hashes the class attributes. I am looking for something similar.

There is hash function for the basic types (list is on std::hash). You need to provide hashing function for your custom type. You can take an inspiration from the cppreference or create an own hashing function.