How to serialize a json object without enclosing it in a sub-object using Cereal

Let's say I have a class in C++, like the following:

struct Point {
    int x, y, z;
};

I want to use Cereal to serialize that struct to JSON. So I added a serialize function like this:

struct Point {
    int x, y, z;
    template<class Archive>
    void serialize(Archive& ar) {
        ar(CEREAL_NVP(x),
           CEREAL_NVP(y),
           CEREAL_NVP(z));
    }
};

This works fine when the Point is a member of another object or an element of an array. But if I want to make the Point be the primary object of an entire JSON file, it doesn't work properly. For example, with the following code:

Point p { 1, 2, 3 };
cereal::JSONOutputArchive ar(std::cout);
ar(p);

I get the following output:

{
    "value0": {
        "x": 1,
        "y": 2,
        "z": 3
    }
}

I'd like to remove the "value0" key and elevate the object to occupy the entire file, like this:

{
    "x": 1,
    "y": 2,
    "z": 3
}

The only way I can seem to do that, is to basically re-implement the serialization function, manually adding the key names.

Point p {1, 2, 3};
cereal::JSONOutputArchive ar(std::cout);
ar(cereal::make_nvp("x", p.x),
   cereal::make_nvp("y", p.y),
   cereal::make_nvp("z", p.z));

Is there any way to do it utilizing the serialize function that I already implemented for the class?

Okay, figured it out. Pretty simple, just needed to call the serialize function directly from the object, passing the archive, instead of passing the object to the archive.

Point p {1, 2, 3};
cereal::JSONOutputArchive ar(std::cout);
p.serialize(ar);