Re: Safety of the Booch Ada 95 Components

["Matthew Heaney" <matthew_heaney@acm.org>]

In article <t7g0x64nw6.fsf@calumny.jyacc.com> , Hyman Rosen 
<hymie@prolifics.com>  wrote:

>> If you don't like that, then don't give me a broken assignment operator.
>
> Why is an assignment operator that raises an exception broken?

Perhaps the case is better stated in terms of the postcondition of the
copy.  If item assignment raises an exception during a copy of a data
structure (which contains zero or more items), then

1) You can implement Copy without handling item assignment errors.  This
is the most efficient way to implement copy, but it will leave the
target data structure in an unknown state.

2) You can implement Copy so that if item assignment raises an
exception, then the target data structure is cleared or only a partial
copy.  This is slightly less efficient than (1) -- unless your compiler
implements zero-cost exception handlers -- but it has the virtue of
guaranteeing the target state.

3) You can implement Copy so that if assignment raises an exception,
then the target data structure is rolled back to its original state
prior to the call.  This is much less efficient than (1), but has the
virtue of leaving target state unchanged.

So take your pick.  As with all system design problems, there is no such
thing as a free lunch, and you're going to have to live with some
compromise.

--
It is impossible to feel great confidence in a negative theory which has
always rested its main support on the weak points of its opponent.

Joseph Needham, "A Mechanistic Criticism of Vitalism"