Re: throwable .vs. non throwable?

 James Kanze <>
Wed, 25 Jul 2007 07:37:06 -0700
On Jul 25, 2:07 pm, (Roland Pibinger) wrote:

On Tue, 24 Jul 2007 12:34:26 -0600, Jerry Coffin wrote: says...

But being able to
specify exception guarantees in code (and not merely in the
documentation) definitely is an advantage.

How? When? Please show us a real example.

class DataBase {
// ...
  Connection* getConnection (...) throw (SQLException);


Without more details, that looks more like an example of where
they don't work, and should be avoided. (I'd be very surprised
if a function as complex as getConnection could not throw
anything but SQLException---that it couldn't throw bad_alloc,
for example.) From appearances (admittedly superficial, but all
I've got to go on here), you are trying to guarantee that
getConnection will throw SQLException in case it detects an
error (and can't create the connection). Regretfully, exception
specifications can't and don't guarantee that; all it says (as a
comment) is that the function might throw an SQLException,
under some undetermined conditions (but then again, it might
not). All it really says is that regardless of what happens,
this function guarantees that it will not throw any other type
of exception---generally, not a very useful bit of information.

I was strongly in favor of
exception specifications until I really tried to use them. I even used
them sometimes for a while -- but I've yet to see a single situation
where they really accomplish anything useful.

They are part of the contract between implementor and user. In this
respect they are very useful.

Becareful about interpreting the contract, however. It's a
negative contract: the function will not throw anything not
listed, and not that it will throw what is listed (much less
that it will throw it when it should). Just knowing that
DataBase::getConnection might throw SQLException, under some
unspecified conditions, doesn't tell me enough to be useful for
anything; I still need the documentation concerning the
conditions under which it will throw this exception. And once
you've documented that, the extra exception specification
doesn't add any new information.

James Kanze (GABI Software)
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