DIP4: Properties

[BLS]

Michiel Helvensteijn wrote:
> BLS wrote:
> 
>>> I think people got it. But it's not a property. Your one-liner seems to
>>> be equivalent to a field. Except, I guess, that you can't take the
>>> address.
>>>
>>> The whole idea of a property is that it can have non-trivial
>>> getter/setter functions. Like a read-only property of a Line, that
>>> returns its length automatically calculated from its two points. Or a
>>> getter and setter that keep a log of all accesses to the information.
>>>
>>> My favorite example is of a Color class, that internally stores its value
>>> in the RGB model, but has properties to read and change its value through
>>> the HSV and HSL models as well.
>> ...
> 
> Again, what is the difference between your one-liners and simple fields?
> 
> immutable property uint theAnswer = 42;
> bool has_cojones;
> 
> A property is much more sophisticated. The whole point is to write your own
> setter and getter methods. Only in very few cases do you want a property
> with simple field semantics. I will give you that Line class I was talking
> about. Tell me how you can do this with your one-liner:
> 
> class Line {
>     private:
>         
>         Point _a;
>         Point _b;
>     
>     public:
>         
>         this(Point a, Point b) { _a = a; _b = b; }
>         
>         property float length {
>             auto get() {
>                 float absX = abs(_a.x - _b.x);
>                 float absY = abs(_a.y - _b.y);
>                 return sqrt(absX * absX + absY * absY);
>             }
>         }
>         
>         // insert trivial properties to read/write points
> }
> 
> You see, the length property doesn't have its own storage. It deduces its
> value from the points of the line.
> 

I don't agree .
1- In most cases we have to deal with simple field semantics, No ? x,y,z
so : immutable property uint theAnswer = 42; is a getter()...... period.


your calculated length property is an exception, should be written in 
standard D code.

IMO a property is a simple thing, a color, a weight, or a mass...