I once worked maintenance on a large C++ program used in production by a lot of customers. It was odd in several ways, but the feature that stands out in my mind was the numerous classes that were not defined anywhere in the source code or libraries.

If that sounds unlikely to you, it sounded unlikely to me, too. I wasted a lot of time trying to figure out where they were defined. I couldn't ask the original author of the code; he had moved on.

Eventually, I found them, sort of. They were being defined by a sed script that ran during the build process. It read the sources before they got to the compiler, constructed class definitions on the fly, and injected them into the code before it was fed to the compiler. So the definitions were right there in the code that the compiler saw; they just weren't anywhere in the code that humans could see.

Why was it done that way? I have no idea.