[vm/ffi] Disallow user methods on Structs

[dcharkes]

Structs are really a view on top of memory. They do not have constructors, their instances are invented by the VM in the FFI. Those instances happen to implement the abstract interface of the struct. They do not have normal fields, only the getters and setters to C memory. They cannot be sub typed.

However, currently we do allow methods on those classes. But they add nothing over extension methods from a user point of view, because they work based on the core interface, and we do not allow sub typing. But they do add complexity.

[mqnfred]

Hello @dcharkes,

I am currently working on a rust <-> dart ffi conversion tool. The goal is to generate a dart API based on a rust API, to consume through ffi. I want to provide an idiomatic dart API which hides ffi concepts such as ffi.Pointer indirections etc.

It is very handy to be able to give methods to classes which extend ffi.Struct. I have in mind, for example:

• Some kind of allocate factory as a constructor
• Some kind of free factory to free the memory behind the reference
• Some cool stuff like getters on ffi.Pointer<Utf8> that return Strings

I do not know if the ability to add methods is inherently an anti-pattern, but I sure know it is convenient. I am guessing that if it went away, I would need to wrap all my API's objects, which feels inefficient and bloat-y.

Here is an example of generated code:

class Coordinate extends ffi.Struct {
        ffi.Pointer<Degree> _latitude;
        Degree get latitude => _latitude[0];

        ffi.Pointer<Degree> _longitude;
        Degree get longitude => _longitude[0];

        factory Coordinate.malloc(Degree latitude, Degree longitude) {
                return mallocCoordinate(latitude.addressOf, longitude.addressOf).ref;
        }
        void free() { freeCoordinate(addressOf); }
}

I am not extremely familiar with dart yet, could you provide an adapted example of this using extension methods and the like?

[dcharkes]

@mqnfred you can just make it all extension methods.

class Coordinate extends ffi.Struct {
        ffi.Pointer<Degree> _latitude;
        ffi.Pointer<Degree> _longitude;
}

extension CoordinateMethods on Coordinate {
        Degree get latitude => _latitude[0];

        Degree get longitude => _longitude[0];

        factory Coordinate.malloc(Degree latitude, Degree longitude) {
                return mallocCoordinate(latitude.addressOf, longitude.addressOf).ref;
        }
        void free() { freeCoordinate(addressOf); }
}

Does this work for you?

[mqnfred]

Thank you very much!

This is great, I was not expecting every type of function/methods to transfer so seamlessly. I now see why the issue is relevant, as we can dedicate the class extending ffi.Struct to being ffi data.