Re: exception handling anti-patterns in documentation

From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2022-05-29 23:59:14

On Sun, May 29, 2022 at 3:49 PM Gavin Lambert via Boost <
boost_at_[hidden]> wrote:
>
> On 30/05/2022 02:10, Robert Ramey wrote:
> > On 5/28/22 3:39 PM, Andrzej Krzemienski wrote:
> >> try {
> >>>    lib::function();
> >>> }
> >>> catch(lib::exception const&) {
> >>>    // handle failure
> >>> }
> >
> > Could you explain what's wrong with the above code?
>
> I believe the point was that if exceptions are always caught immediately
> then it smacks of the "exceptions as control flow" anti-pattern, and a
> non-throwing form of the library function ought to be available/used
> instead.  Essentially, the exception is being used as an alternative
> return value rather than as an actual exception.
>
> It's from the school of thought that exceptions should be "truly
> exceptional" -- i.e. any condition that might theoretically raise an
> exception can be explicitly checked in advance by a non-throwing method

I'll let him speak for himself, but I'd be surprised if Andrzej's point
comes from that school of thought, he is not a Rust programmer. :)

The issue with such examples is that they are useless at best: an
experienced programmer knows what to do, while a novice might think that he
is always supposed to immediately catch exceptions, as may be the case in
other languages (e.g. Java) which lack deterministic termination.

The correct use of exception handling in C++ is to let exceptions propagate
unmolested by default; there is no explicit checking for errors after
calling a function that may throw. You catch only where the error can be
handled and the program can restore normal operations. And while it is true
that unhandled exceptions terminate the program, that is always a logic
error, programs should handle all exceptions that may be thrown.
Terminating the program when an exception is thrown is valid only under
-fno-exceptions (in Boost you'd do that by providing a suitable definition
for boost::throw_exception).