This is a question not addressed in the cheatsheet, as far as I can tell... I asked a question two weeks ago on the forum, but haven't seen any responses there -- sorry for the repeat posting.

I have a C++ class member that holds a callback function pointer. It is a std::function, so it's valid for it to be empty (nullptr). I have no problems binding the class in ChaiScript, but I don't know the proper way to test if that member is empty, nor how to set it to nullptr (or 0).

// Example struct with callback member
struct MyClass {
    std::function<float(float)> callbackFunc = {};
};

// After registering MyClass, register the member:
m->add(fun(&MyClass::callbackFunc), "callbackFunc");

In scripting, I am able to create an instance of my class, assign a lambda function to it, and evaluate the function (e.g. using obj.callbackFunc(3.1f)). But I don't know how to check if it's empty, or how to reset it to nullptr (or 0). For the empty (boolean) test, I tried adding a type conversion:

chai.add(type_conversion<decltype(MyClass::callbackFunc), bool>([](decltype(MyClass::callbackFunc) f) { return f != nullptr; }));

That doesn't work, I get a "Condition not boolean" when trying to test the member. For the assignment, I tried various permutations of operator= (taking an int argument, i.e. 0), but haven't yet hit on something that worked.

I can make a specific method to check or reset that member, but I'd rather something that worked "automatically" for any type of std::function variable.

For the emptiness test, I came across this in the docs:

obj.callbackFunc.is_var_null()

but that doesn't seem to work (it returns false whether or not my callbackFunc is set to something), presumably because std::function is an object and not (directly) a pointer...

Any suggestions?

Desired Behaviour

obj = MyClass();
obj.callbackFunc = fun(x) { return x * 13.5f; }; // callbackFunc is a std::function member defined in C++

if (!obj.callbackFunc) {
    print("callback not set");
}
else {
    print(obj.callbackFunc(3.1f));
}
obj.callbackFunc = nullptr; // or "obj.callbackFunc = 0" would be fine