I was quietly reading this thread, but I must respectfully but strenuously
disagree with the premise that exceptions are "simply another kind of flow
control". OK, semantically exceptions _are_ another type of flow
control. Let's not jump down a rathole. But, in any language,
exception-safe code is much harder to write than most programmers realize.
If every method is re-entrant, there is nothing to worry about because
stack unwinding deals effectively with method-local state. But as soon as
methods start affecting the longer term running state of a system,
exceptions can easily leave that system in an inconsistent, possibly broken
state.
True, bad code can do all that without the "benefit" of exceptions. But,
even otherwise solid code can go horribly wrong because exceptions are
thrown in a 3rd party library. Non-throwing code is much, much less likely
to have that impact.
This is the reason why coding standards for systems with very high uptime
requirements often disallow throwing exceptions. This can extend to
disallowing use of libraries that throw (or taking pains to configure libs
so that they do not).
Put another way, the "rare" nature of Exceptions (go figure) is implied in
the sub-optimal treatment given by compiler writers to exception code
paths. You sure as heck don't want to optimize Exceptions at the expense
of the regular, non-Exception code paths!
At 12:03 PM 3/17/2010 -0400, John Redford wrote:
Which begs the answer to your question: Yes, people use die as
goto. People use longjmp as goto. People use throw as goto. Exceptions
have an association with concepts like "error case" and "should rarely
happen"; but that is entirely wrongheaded. Exceptions are simply another
kind of flow control. There are some low-level implications in terms of
CPU efficiency, but it's just a matter of unwinding the stack until a
point is found where some code should run, and then adjusting the
instruction pointer. Within a single stack frame, it reduces to simply
being a goto. Some languages (Perl, Java, others...) approach exceptions
as "rare" and so have implementations that deter their use in
high-performance scenarios -- so it's often a question not of program
structure, but optimization and
frequency.
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