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From: David R Tribble <david@tribble.com>
Subject: Re: ++var and throw
Date: 1999/09/27
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Ed Brey <brey@afd.mke.etn.com> writes: 
>> In the spirit of the ++var thread, I have a spin-off question (I
>> coincidentally just ran into this while doing real work).  Consider
>> the statement:
>> 
>> *p++ = f();
> 
>> If f() throws, is p guaranteed to not be incremented?  Where does
>> the IS define this or say that it is undefined?
>> ...
>> I hope that does not mean that it can be interrupted by the throw
>> of an exception because that could leave p in an unstable state.

Steve Clamage wrote:
> The expression statement has two side effects:
> 1. assignment to *p
> 2. modifying p by incrementing its value
> 
> Side effects cannot be delayed beyond reaching the applicable
> sequence point. Whether completion occurs sooner than the sequence
> point is unspecified.  [...]
> 
> You therefore should not write code that depends for correctness
> on multiple side effects in expressions, if exceptions can occur.

Exactly; don't do it.  Do something instead that will leave p in a
well-defined state, such as:

    *p = f();    // might throw, p is unchanged
    p++;         // now p is changed safely
                 //  (assuming p is not an iterator)

When you play with fire (exceptions), expect to get burned once in
a while.

-- David R. Tribble, david@tribble.com --


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