And even then, I've often heard it stated as "zero cost if you don't use it", eg, exceptions have zero runtime cost unless an exception is actually thrown, so, given that they should only be thrown in exceptional cases, that really shouldn't impact your program. (Whether or not its truly zero cost depends, I guess, eg does memory count as cost? What if it makes something not fit in cache?)

I love that exceptions are the classic case for this, because it's not even true that "zero-cost exceptions" are zero _runtime_ cost on the non exceptional path. The most trivial example is they block vectorization.

Well, exceptions shouldn't be the classic case here, because - as you say - they're typically not zero cost.

Zero-cost abstraction is what you get when your abstraction gets compiled away to nothing. Like properly written value wrappers in C++, or (presumably) newtype in Rust. These things disappear from final assembly.

The definition of zero-cost abstractions, as introduced in C++, was that "it can't be slower that code than the equivalent hand-written code" (e.g. code not using the abstraction).

In that regard, exceptions are interesting as if you're on the happy path (which should be 99.999% of the time - a normal program that uses exceptions as error handling method should not encounter any exception if you do a `catch throw` on an average run of the software), they can cost less than return-value-based error handling (https://nibblestew.blogspot.com/2017/01/measuring-execution-...). If you're on a "sad path", though, they will cost more.

What is pretty sure is that since compilers learned to put the "sad" path in .cold section, the code size issue has become a 100% non-issue, the "sad" path won't bloat the hot, exception-less path ; in my experience, exceptions are in cases that matter a negative-cost abstraction.

As always with microbenchmarks...

  struct Error* create_error(const char *msg) {
    struct Error *e = malloc(sizeof(struct Error));
    e->msg = strdup(msg);
    return e;
  }

Is it measuring exceptions vs return codes, or creation cost of std::runtime_error(const char *) (small string opt?) vs malloc() + strdup?