ClassMetadata layout

[NSExceptional]

@Azoy How did you come to this class metadata layout structure?

struct _ClassMetadata {
  let _kind: Int
  let _superclass: Any.Type?
  let _reserved: (Int, Int)
  let _rodata: UnsafeRawPointer
  let _flags: ClassMetadata.Flags
  let _instanceAddressPoint: UInt32
  let _instanceSize: UInt32
  ...
}

After looking through objc-runtime-new.h I was able to come up with a working structure in CXX for determining when an object was defined in Swift, but I don't know how you came up with the name rodata for example, or where the rest of the fields below it come from. I don't see them in the definition for objc_class. Are the rest Swift-only or something?

[Azoy]

So class metadata is a little weird for ObjC. The first 4 fields represent struct objc_class: objc_obejct { ... } where:

struct _ClassMetadata {
  let _kind: Int                // Class isa
  let _superclass: Any.Type?    // Class superclass
  let _reserved: (Int, Int)     // cache_t cache
  let _rodata: UnsafeRawPointer // class_data_bits_t bits
  // EVERYTHING else is Swift specific and not defined for ObjC classes.
  // So like getting the descriptor in an ObjC class will result in a runtime crash.
}

I'm unsure exactly where I got the name rodata from for this field, but in reality this is the rwdata pointer after class realization. This is actually the type of data being stored here however. So in a binary this field is the pointer to rodata, but the ObjC runtime has full control over these fields and replaces it with rwdata.

[NSExceptional]

I see it used in the objc runtime source code, I wonder what it stands for. Oh well. Thanks :)