How can I create custom hash functions in C++17?

How can I create custom hash functions in C++17?

In C++17, you can create custom hash functions by defining a struct that implements the operator() method. This allows you to specify how to hash your custom types.

Can you show me an example of a custom hash function?

Sure! Here's a simple example for a struct called Point:

struct Point {
    int x, y;
};

struct PointHash {
    std::size_t operator()(const Point& p) const {
        return std::hash<int>()(p.x) ^ std::hash<int>()(p.y);
    }
};

This combines the hashes of the x and y coordinates.

What about using this hash function in an unordered_map?

You can use the custom hash function in an unordered_map like this:

std::unordered_map<Point, std::string, PointHash> pointMap;
pointMap[{1, 2}] = "Point A";

This allows you to use Point as a key in the map.

Are there any performance considerations I should be aware of?

Yes, when designing a hash function, aim for a good distribution of hash values to minimize collisions. A poor hash function can lead to performance degradation in hash tables.