Enforce either Full or Empty Inner Object in JSON Schema

[StormLord07]

We have two structs, Inner and Outer, defined as follows:

struct Inner {
    std::string x;
    std::string y;
};

struct Outer {
    int x;
    Inner inner;
    int y;
};

We want to support only JSONs that have the inner object either fully populated or completely empty. Specifically, we want to support:

{"x": 1, "inner": {"x": "hello", "y": "world"}, "y": 2}

or

{"x": 1, "inner": {}, "y": 2}

But not:

{"x": 1, "inner": {"x": "hello"},"y": 2}

or

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

The goal is to ensure that the inner object is either fully populated or completely empty, but not partially populated and should exist. Any suggestions on how to achieve this?

As far i've read into optional but it doesnt seem like its the way because it will make it so if the inner is not in schema it will be accepted.

[stephenberry]

This is related to this issue: #1821. However, the constraint you want is supported because it is a constraint on your inner struct.

Full demo: compiler explorer link

#include <iostream>

#include "glaze/glaze.hpp"

struct Inner {
    std::optional<std::string> x;
    std::optional<std::string> y;
};

struct Outer {
    int x;
    Inner inner;
    int y;
};

template <>
struct glz::meta<Outer> {
    using T = Outer;
    static constexpr auto inner_constraint = [](const T&, const Inner& inner) {
        if (inner.x && inner.y) {
            return true;
        } else if ((not inner.x) && (not inner.y)) {
            return true;
        }
        return false;
    };

    static constexpr auto value = object(
        &T::x,  //
        "inner",
        read_constraint<&T::inner, inner_constraint, "Both x and y must be included or excluded!">,
        &T::y);
};

int main() {
    Outer obj{};
    std::string buffer = R"({"x": 1, "inner": {"x": "hello"},"y": 2})";
    auto ec = glz::read_json(obj, buffer);
    if (ec) {
        std::cout << "Sucessful constraint error:\n";
        std::cout << glz::format_error(ec, buffer) << '\n';
    }
    buffer = R"({"x": 1, "inner": {"x": "hello", "y": "world"}, "y": 2})";
    ec = glz::read_json(obj, buffer);
    if (ec) {
        std::cout << glz::format_error(ec, buffer) << '\n';
    }
    else {
        std::cout << "Success!\n";
    }
    return 0;
}