A really common pattern in many programs is to associate metadata with something using a unique key. In cases where you can’t associate that metadata directly

A really common pattern in many programs is to associate metadata with something using a unique key. In cases where you can’t associate that metadata directly using a pointer (by using structures as keys embedding the pointer in the key) or a direct index into a metadata array, a perfect hash generated at build time is about as good as you’re going to get. The other thing is that often this is easy to bolt on after the fact. As an example, in network devices it is common for a syslog mnemonic to be associated with an instance of a rate limiter. A good solution is to compile your log messages using an errmsg definition and compiler tool (which also allows you to build documentation) but this requires a lot of up-front planning and build orchestration and makes the code awkward in sone circumstances. An alternative is that you allow people to declare their mnemonic (and, for example, rate limit) in-place and then allow the logger library to use that to look up other properties that are joined from other sources (if there are any) and glean the structures that need to be created. Since extracting the full set on mnemonics is necessary anyway (for tripwires, for documentation, ...) you can now build the hash function after-the-fact. This definitely simplified the build considerations (and avoids unnecessary rebuilds when, for example, only documentation changes). The after-the-factness-but-with-low-overhead part is what’s nice about it. Yes, it’s better to structure things up front to make the system optimal but it’s nice to be able to get close.