C++ – Do Preprocessor Defines Slow Down Build Times?

They can, as shown by new languages that do.

But a design decision was made all those years ago (when the C compiler was multiple independent stages) and now to maintain compatibility the pre-processor has to act in a certain way to make sure old code compiles as expected.

As C++ inherits the way it processes header files from C it maintained the same techniques. We are supporting a old design decision. But changing the way it works is too risky lots of code could potentially break. So now we have to teach new users of the language how to use include guards.

There are a couple of tricks with header files were you deliberately include it multiple times (this does actually provide a useful feature). Though if we redesigned the paradigm from scratch we could make this the non-default way to include files.

Ok, I would never in a million years do this, but you could do something like this:

Force auto inclusion of this:

// forcebanned.h
#define for "You must include banned.h"

Then add this:

// mybanned.h
#undef for
#include banned.h

But don't do that. Better is to make the inclusion of banned.h a coding standard, and rely on code reviews to catch it.

If you are really insistent on enforcing this technically, you could have a custom build step that grep'd files for "banned.h" prior to compilation, failing the build if the line isn't found. (Or use some static analysis tool that tags deprecated functions.) Doing so as part of the build keeps your code simpler, and more portable.