How to get pointer from embedded to owner?


#1

I have structure A which have embedded structure B. Sometimes struct B must get field of A.
For example struct Widget has field Font. Implementation method of Font calls CreateFontIndirectW and next SendMessageW with owner’s field hwnd. I can’t “font : Font::new(&self)” in constructor because constructor has not field self;
Similar problem of lack self in constructor is settings c-arrays: I have lfFaceName: unsafe { mem::zeroed() }, whereas better would be:
let face_name_ptr = (&mut font_struct.lfFaceName).as_mut_ptr();
to_array(face_name_ptr, winapi::LF_FACESIZE, “Arial”);
but how to make in constructor?


How to project event driven library?
#2

There are no constructors in Rust. new() is just a convention for a regular function, so if you have struct Font, then Font {} is the instance of it.

impl Widget {
   fn new() -> Widget {
      let widget = Widget {font: None};  // or use a default font if you don't want Option<Font>
      let font = Font::new(&widget);
      widget.font = Some(font);
      widget
   }

Please keep in mind that in Rust a struct is not allowed to hold a reference to itself (nor reference to anything in itself), so if Widget contains a Font, then Font is not allowed to keep a reference to the Widget. When you compose structs use only owned values, or Rc if you can’t avoid parent references.