Dr. Dobb's is part of the Informa Tech Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Channels ▼


Expression Generics

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.


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.

Related Reading

More Insights

Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.