Why use C++?

[James Kanze]

And the "bad"
techniques, like just typing in code without having done any
design, don't really make such a difference in very small
programs.
I still sort of [design] I just don't write it down.

You don't "know" something until you've written it down. It's
too easy to cut corners otherwise.

The Source Code IS The Design!

The source code is an implementation of a design (or of a lack
of design).

http://www.developerdotstar.com/mag/articles/reeves_design.html

I was really talking about 300 line filters. I do think about their
structure but I don't draw a UML diagram or anything.

A design doesn't have to be a UML diagram. Depending on the
program's complexity, it might not be much more than a man page,
stating exactly what the program is going to do.