[Phoenix-pm] Fwd: ++ turns undef into 0 first

Scott Walters scott at illogics.org
Tue Sep 6 14:11:47 PDT 2005

Here's an interesting gotcha sent to the perl-porters list:

----- Forwarded message from Peter Scott <Peter at PSDT.com> -----

From: Peter Scott <Peter at PSDT.com>
Subject: ++ turns undef into 0 first

This recently bit me:

$ perl -wle 'print 42 if defined($h{foo}++)'

Now, before thousands of you reach for the flamethrowers, yes, I know
- now - that this is documented.  I found it in perlop right where it
ought to be:

| undef is always treated as numeric, and in particular is changed to 0
| before incrementing (so that a post-increment of an undef value will
| return 0 rather than undef).

The only thing that perturbs me is that I had to find this after I was
starting to wonder whether there was a bug.  So this looks to me like
suboptimal behavior that has been documented; any chance of making it
behave the more logical way, i.e., that the expression evaluates to the
original value and not some modified one?  I can't imagine there is much
code relying on the current behavior.

Peter Scott

----- End forwarded message -----

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