Implement index operators for Arrays

[BastiaanOlij]

Resubmit of GDExceptions array index operators

[BastiaanOlij]

Original PR on Vnens repo: vnen#23

note that we're still looking into why Variants are giving issues.

[alessandrofama]

interface needs to be renamed to gdn_interface based on #644 ? (just pulled master and was already using the changes in this PR to test my GDExtension).

[BastiaanOlij]

@alessandrofama Long overdue to rebase, let me clean it up :)

Was it working for you? I was having issues with the Variants themselves before but that may have been resolved by now.

[alessandrofama]

It was not working, but at least I could compile while working on other stuff :D

While looking at the debugger, I noticed that the address of the returned Variant* and Array* (this) were the same here:

godot-cpp/src/variant/packed_arrays.cpp

Line 134 in c2b6904

Variant *var = (Variant *)internal::gdn_interface->array_operator_index((GDNativeTypePtr *)this, p_index);

Thought that was strange, so stepping through Godot's source, it seems to me that Array::operator[] is not being called before returning the void pointer here:

https://github.com/godotengine/godot/blob/4387f9645b1f54755506804770ba15c6c9cd5094/core/extension/gdnative_interface.cpp#L774-L778

Weirdly enough, calling the operator directly seems to have fixed the crashes for me:

return (GDNativeTypePtr)&self->operator[](p_index);

Hobby dev here, so not sure why it would behave like this 🤔 (might be completely wrong here, have been a gdnative user for a while, but new to the source).

[BastiaanOlij]

That actually makes a lot of sense, what it is likely doing is taking &self first, which is a pointer to the self pointer, and then moving that pointer forward by index. So return (GDNativeTypePtr)&(self[p_index]); would solve it too.
The operator[] syntax might be more clear...

[BastiaanOlij]

Once the fix in Godot is merged, we can merge this. Works like a charm now, thanks @alessandrofama !

[BastiaanOlij]

@akien-mga with the fix in Godot merged, I think we can merge this too, unless we want people to verify if it works first? it worked for me so :)

[akien-mga]

Thanks!