DIP4: Properties

[Michiel Helvensteijn]

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.

-- 
Michiel Helvensteijn