I only have one comment : the title “Don’t use magic numbers” and the use of the expression “magic numbers”, because for me a “magic number” is a specific number (as 0xAB12CD89) used to check a structure of data in memory (for example a hard drive partition structures start) to prevent as much as possible to interpret corrupted data (which often leads to coredump due to overflow, when using length fields in the corrupted data).

As the first sentence says in your paragraph, what you are talking about, for me, is called “hard-coded numbers/values” (in French, as I am, “valeurs en dur dans le code”).

In summary, C++ continues to be a complex language to master, but there are tools out there that can act as the “guy looking over your shoulder” pointing out what you’re missing, or when you’re applying a potentially damaging semantic or side-effect.

Still, if you have a specific, one-off problem, making the solution a reusable library is a waste of time, or maybe even a source for a maintenance nightmare. Writing a good reusable library is difficult, and often it takes a couple of rewrites and revisions, plus more understanding of the problem domain than might be initially available, to make it truly usable.

Now, if your fingers start itching to make a Copy/Paste of the original one-off, it obviously isn’t a one-off any more, but a candidate for a reusable library. At that time it might be best to start from scratch and design the library properly, then maybe replace the original code. Not the other way round.

The subnote on item #2 is funny. I’d say iii (or one of the classic alternatives i, j, k and m) is a perfect name for a loop variable. It is sensible to give better names to other variables, of course.