February 2012


Representations of rectangles and points are essential in any system working with graphics. Every API and many applications define their own. Depending on the era of the API there may be a full suite of algebraic-style operations, or the types may be simple data carriers with a handful of manipulating functions provided.

So what happens when bodies of code collide? Well, we end up with ad hoc transference of information from one representation to another, and chunks of code using a rich representation just look different from chunks using something more primitive.

In ZooLib I made several attempts at creating a clean suite of cartesian types and operations, with conversion operators and constructors, pseudo constructors and miscellaneous helper functions. But the approach just never scaled well.

Recently I've taken a very different tack. In ZCartesian I've defined a suite of template accessor functions -- passed a rectangle/point the edges, corners and centers can be extracted. Offsetting and aligning operations build on those accessors, as do sensible algebraic operators.

The accessor functions themselves forward to functions defined in traits templates. There's a suite of base classes for rectangles that are defined by variations of origin/extent or left/top/right/bottom. Supporting a new suite of cartesian types can be as simple as providing the definition of a traits class inheriting from one of the provided bases.

In this way we can work with NSRect, CGRect, XRectangle, QuickDraw Rect, Windows Rect, SDL_Rect, and their associated point (and size) structures using identical notation. And when we need to switch representations there's just a single pseudo-constructor to do the job.

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