Item 14: Use exception specifications judiciously

There’s no denying it: exception specifications have appeal. They make code easier to understand, because they explicitly state what exceptions a function may throw. But they’re more than just fancy comments. Compilers are sometimes able to detect inconsistent exception specifications during compilation. Furthermore, if a function throws an exception not listed in its exception specification, that fault is detected at runtime, and the special function unexpected is automatically invoked. Both as a documentation aid and as an enforcement mechanism for constraints on exception usage, then, exception specifications seem attractive.

As is often the case, however, beauty is only skin deep. The default behavior for unexpected is to call terminate, and the default behavior for terminate is to call abort, so the default behavior for a program with a violated exception specification is to halt. Local variables in active stack frames are not destroyed, because abort shuts down program execution without performing such cleanup. A violated exception specification is therefore a cataclysmic thing, something that should almost never happen.