Zig dynamic dispatch with @fieldParentPtr

main.zig

const print = @import("std").debug.print;

const Animal = struct {
    const Self = @This();

    speakFn: fn(*const Animal) void,

    pub fn speak(self: *const Self) void {
        self.speakFn(self);
    }
};

const Dog = struct {
    const Self = @This();

    animal: Animal,
    call: []const u8,

    fn init() Self {
        return .{
            .animal = .{
                .speakFn = speak,
            },
            .call = "bark!",
        };
    }

    fn speak(animal: *const Animal) void {
        const self = @fieldParentPtr(Dog, "animal", animal);
        print("{}\n", .{self.call});
    }
};

const Cat = struct {
    const Self = @This();

    animal: Animal,
    call: []const u8,

    fn init() Self {
        return .{
            .animal = .{
                .speakFn = speak,
            },
            .call = "meow!",
        };
    }

    fn speak(animal: *const Animal) void {
        const self = @fieldParentPtr(Cat, "animal", animal);
        print("{}\n", .{self.call});
    }
};

fn doStuff(animal: *const Animal) void {
    animal.speak();
}

pub fn main() void {
    const dog = Dog.init();
    doStuff(&dog.animal);
    const cat = Cat.init();
    doStuff(&cat.animal);
}

Your cat has an identity crisis.

Nice, haha. The perils of manual vtables.