(Maybe) Strange Behaviour of Field Initialization

[Adam D. Ruppe]

On Wednesday, 28 April 2021 at 15:09:36 UTC, eXodiquas wrote:
> ```d
> class Particle : Drawable
> {
>   CircleShape shape = new CircleShape(5);

This `new` is actually run at compile time, so every instance of 
Particle refers to the same instance of CircleShape (unless you 
rebind it).

>     this.shape.position = Vector2f(x, y);
>     this.shape.fillColor = Color.Green;

Which means each constructor here overwrites the fields on the 
same object!

What you probably want is to construct the shape in the 
constructor too, since that is run for each instance created, 
instead of just once at compile time and reused for each instance.

This behavior D has is pretty useful... but also pretty 
surprising to people coming from other languages. I kinda wish 
the compiler made you be a little more explicit that you actually 
did intend to compile time construct it.

> The last time I wrote something in D is a few months back, but 
> I cannot remember this behavior. In the code above `shape` acts 
> like as if it were `static` without being `static`, obviously.

Well, the instance is static, but the handle is not.

If you were to do a `this.shape = new Shape;` then that object 
would refer to a new one, but if you don't it all uses the same 
object.

The code you have is kinda like if you wrote:

static Shape global_shape = (at ctfe) new Shape;

(in the instance) Shape shape = global_shape;