An Imaging Example
A bitmap can be represented as a vector of pixels, with additional attributes describing height, width, pixel format, the color palette, and so on. As you can see, expression generics can optimize operations OR, AND, XOR, or taking the one's complement. To illustrate the idea, I wrote a sample program. Class Vector is replaced by class Bitmap that initializes its container from a JPEG file. When using WPF classes, a plain byte array is better suited for bitmap manipulations than a List<> that I used in the Vector class. Consequently, class Bitmap needs its own iterator implementation. I also need to carry image attributes from the leaves to the root where they are used to render the resulting image on the screen. Struct BitmapParams serves this purpose and interface IIter is replaced with IBitmap that knows about BitmapParams. All expression nodes implement this interface, and the BitmapParams instances are copied from the leaves to the root. To simplify the illustration, all input images have the same attributes. The expression is evaluated when the root node is converted back to a Bitmap.
Conclusion
High-level languages offer greater productivity, but writing efficient code is challenging, since the programmers are farther removed from the machine. Sometimes we have to trade productivity for the runtime advantages of a lower-level language, or accept performance limitations. Expression templates and similar techniques give us a chance to avoid this compromise and write faster and more compact code.
