(Maybe) Strange Behaviour of Field Initialization

[eXodiquas]

On Wednesday, 28 April 2021 at 15:35:57 UTC, Adam D. Ruppe wrote:
> 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;

Thanks, this is a pretty good explanation. I get it now. :)

This behavior sounds pretty neat, as long as you know about it. :P