Opaque Type Heirarchies

new_zig_feature.zig

pub const Animal = *opaque {};
// allow opaque to take a type as a parent
pub const Dog = *opaque(Animal) {};
pub const Cat = *opaque {};

const some_library = struct {
    pub extern fn makeDog() Dog;
    pub extern fn bark(Dog) void;
    pub extern fn pet(Animal) void;
};

pub fn main() void {
    const dog = some_library.makeDog();
    some_library.bark(dog);
    // no need to cast because a Dog was defined as an Animal
    some_library.pet(dog);
}

wrap_ptrcast.zig

pub const Animal = *opaque {};
pub const Dog = *opaque {
    pub fn asAnimal(self: Dog) Animal {
        return @ptrCast(Animal, self);
    }
};
pub const Cat = *opaque {};

const some_library = struct {
    pub extern fn makeDog() Dog;
    pub extern fn bark(Dog) void;
    pub extern fn pet(Animal) void;
};

pub fn main() void {
    const dog = some_library.makeDog();
    some_library.bark(dog);
    some_library.pet(dog.asAnimal());
}