Re: [FWP] Fun with lvalue subs

[sthoenna@efn.org (Yitzchak Scott-Thoennes)]

In article <m13dkhukfy.fsf@rt158.private.realtime.co.uk>,
Piers Cawley <pdcawley@bofh.org.uk> wrote:
> 
> sub my_substr ( $$;$$ ) : lvalue {
>     my $retval;
>     tie $retval, 'Lval', \ $_[0], @_[1,2] or die;
>     $retval = $_[3] if exists $_[3];

Thats a cute way of checking if a parameter was passed, but doesn't
work in all cases.  Internally, exists on an array element (a new
feature with 5.6.0, for those who are wondering) just checks if the
scalar value pointer is to the special global undef value.  Thus it
will not always work, since sometimes an undef value that is pass in
will also actually be the global undef value.  For instance:

#!perl -w
sub e
{ 
    print(scalar @_, " parameter(s) passed.  Parameter 0 ",
          exists $_[0] ?  "exists!\n" : "Doesn't exist!\n")
}
e(undef);  # a value passed but exists is false
e($_) for undef;  # also through an alias
sub ee { e(@_) }
ee(undef);        # including an alias through @_

The moral: if you want to check if a parameter is defined, use
defined().  If you want to check if a parameter was passed, use
scalar(@_).

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