DIP4: Properties

[Bill Baxter]

On Tue, Jul 28, 2009 at 3:53 PM, BLS<windevguy at hotmail.de> wrote:
> 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...

How is length not as simple as a color?  I don't get it.

And consider the case of complex numbers.  Either the pair (real,imag)
is stored or (modulus,argument) is stored.  Neither is more
"fundamental" than the other as a representation.  Which one do you
make a field and which one "standard D code"?

--bb