Question

The common Superclass of Rectangle and Circle is Shape.

If I initialize some shapes, what is a good way of converting the shape into a circle later and keeping the same properties set while it was a shape? Should I implement a initWithShape in the subclasses that looks something like this?

- (id) initWithShape:(Shape*)aShape {
    self = (id) aShape;

    // set circle or rectangle specific values

    return self;
}

Does anyone have an example that I can look at?

Answer 1

You cannot change an object after it has been created, except by freeing it and creating a new one (which you can do in an init method, and is in fact quite often done for singletons or class clusters), but that is not really what you’re after.

Give an existing Shape object, with some properties, your only real option is to create a new object based on the shape properties. Something like:

In Shape.m:

- (id) initWithShape:(Shape*)aShape {
    self = [super init];
    if (self != nil) {
        // copy general properties
        _x = aShape.x;
        _y = aShape.y;
        _color = [aShape.color copy];
    }
    return self;
}

In Circle.m:

- (id) initWithShape:(Shape*)aShape {
    self = [super initWithShale:aShape];
    if (self != nil) {
        // base properties on the class of the shape
        if ( [aShape isKindOfClass:[Oval class]] ) {
            // average the short and long diameter to a radius
            _radius = ([(Oval*)aShape shortDiameter] + [(Oval*)aShape longDiameter])/4;
        } else {
            // or use generic Shape methods
            _radius = aShape.size / 2;
        }
    }
    return self;
}