Add utility to optionalize a struct

[aurelleb]

I don't know if this would be in scope or feasible at all with the way reflection is currently emulated, but here is the idea.

If I have this struct:

struct Meta {
	std::vector<std::string> hobbies;
};

struct Person {
	std::string title;
	int age;
	Meta meta;
}

I could use something like glz::optionalize to turn it into:

struct Person {
	std::optional<std::string> title;
	std::optional<int> age;
	std::optional<glz::optionalize<Meta>> meta; // maybe optionalize could provide an option for whether nested structs get recursively optionalized
}

That way we can just create a partial for any T simply doing using PartialPerson = glz::optionalize<Person>.

This kind of construct is very useful, notably for configuration, where there might be a default configuration that gets merged with a user configuration that only specifies a few values to override. The merging is done just fine with glz::merge, but the ability to produce a fully optionalized type would remove the need to hardcode it and effectively maintain two types. That way partial updates can be done in type-safe way.

Right now, I'm forced to maintain different types for this which is very messy

[stephenberry]

This becomes feasible with C++26 reflection, which is actually well underway. I've recently been testing an experimental version of Glaze that uses Clang with C++26 reflection underneath. My guess is that in a year Clang will be able to do this, and probably GCC as well.

I'm going to keep this is alive, because I like the idea, but I think this is best waiting for C++26 when we can generate structs at compile time.

[aurelleb]

Yeah that makes sense, can't wait for C++26 reflection!