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Postings from September 2013
Linda Walsh via RT <perlbug-followup@perl.org> wrote:
> It didn't "blindly modify" it. The value of $_ was *saved*
> in the code that made use of it and restored when done.
Not exactly; the code restored $_ when done, but only if it returned
without throwing an exception. Your new code has the same problem
(ignoring its minor syntax errors):
> sub reader() {
> my save=$_";
> open(my $fh, "<myfile") or die "open:$!";
> while (<$fh>) {
> ...
> }
> $_=$save;
> }
If the file "myfile" does not exist (or the process doesn't have
permission to read it, or no more file descriptors are available, or a
host of other error conditions), this routine throws an exception (die
"open:$!"). That's almost always the correct thing to do when opening
a file fails, but it means that the rest of the routine doesn't run —
including the cleanup code.
If this code used either "local $_" or a lexical variable instead of
the manual save/restore, it would not have that problem — and nor
would it have problems when the existing value of $_ was read-only.
> That would *seem* to be a perfectly reasonable solution in most
> languages. Even in assembler, if you use registers, you save them,
> you use them, you restore them.
That's a red herring; I'm not aware of a CPU with registers that can
alias a read-only value.
--
Aaron Crane ** http://aaroncrane.co.uk/
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