Programmer Musings

More on Magic Constants

I've been thinking more on the issue of Magic Constants. Have you ever noticed that when some people first understand the idea of symbolic constants, they want to collect all of the constants they've defined together in one spot?

I remember a C project I worked on over ten years ago where the programmer had a single header file with every constant in the system defined in it. He also had a second header with every struct in the system declared in it. This was so "he would always know where they were."

Of course, this meant that any change to either of those headers meant the entire system needed to be recompiled. As a result, creation of new constants (which had to be in the constants header file) was strongly discouraged. This obviously encouraged the misuse of constants for new, unrelated purposes.

When I came to the project, there were already a large number of places where any given constant was used for two or more unrelated purposes, because it happened to have the right value. Some arrays and strings could not be resized because changing a constant would have broken unrelated code. Fixing most of those problems took months.

The funny thing is, I have continued to see this same pattern in almost every language I've worked in since. Why do people think that it is a good idea to build a single constants file? I've done it. Others have done it. Sometimes we do it even when we know better. I wonder if this is similar to the mental quirk that causes us to make kitchen junk drawers and Miscellaneous folders in filing cabinets.