std::variant and JS null

[Reuben Scratton]

First up let me say how much I love this project! It's embarrassingly better than my own feeble attempt to wrap C++ types for use with QuickJS, and I've learned several useful template meta-programming tricks from reading the source code.

I hit a snag when I tried to implement a C++ object property that could be set (in JS) to a string, or an integer, or null. Obviously I chose to implement this property as having type std::variant<std::string, int> on the C++ side. But how to represent null? I tried using void*, shared_ptr, optional... but couldn't get anything to work. RTFM'ing it seems std::monostate is the thing to use, i.e. my property type becomes std::variant<std::string, int, std::monostate>.

To get this to work I had to implement a js_traits that can wrap and unwrap std::monostate and also alter the existing js_traits<std::variant<...>>::unwrap() to use std::monostate when given a JS_NULL.

My question is: Have I done the right thing? Is this the best way to implement support for nullable property values?

[ftk]

I think std::monostate is more similar to js undefined (because js variables are undefined when not initialized). For js null std::nullptr_t might be good choice.

For variant support, yes, you currently need to modify quickjspp.hpp to add new types

[DD]

Just a quick comment to mention that std::variant default-constructs to its first type's default value.
Thus I think typically, std::monostate (or std::nullptr_t) should be your first rather than last type.
And @ftk , very good point about undefined ~= std::monostate and null ~= std::nullptr_t.