On Fri, Oct 19, 2001 at 10:36:38PM -0400, misc wrote:
> now the following code:
>
> $qry = "SELECT * FROM table WHERE field=5";
> $sth = $dbconn->prepare($qry);
> $sth->execute or die($sth->errstr);
> $row1 = $sth->fetchrow_hashref;
> $temp = $$row1{'field'};
> print " got here ** ";
> if ($row1 ne "") {
> print "row not empty";
> } else {
> print "row is empty";
> }
>
> with the same SQL statement and database
> prints out:
> got here ** row not empty
>
> why? why is the reference not undef still
> in the second case? By dereferencing an
> undef reference it becomes defined? Isn't
> that akin to raising the dead? I am running activestate Perl 5.004 under win98se.
>I am
> using DBD::ADO and an access2000 database.
>
This is autovivification. See perlref:
The left side of the arrow can be any expression
returning a reference, including a previous
dereference. Note that $array[$x] is NOT the same
thing as $array->[$x] here:
$array[$x]->{"foo"}->[0] = "January";
This is one of the cases we mentioned earlier in which
references could spring into existence when in an
lvalue context. Before this statement, $array[$x] may
have been undefined. If so, it's automatically
defined with a hash reference so that we can look up
{"foo"} in it. Likewise $array[$x]->{"foo"} will
automatically get defined with an array reference so
that we can look up [0] in it. This process is called
autovivification.
If you don't want to autovivify $row1, do this instead:
if ($row1) {
$temp = $row1->{'field'};
}
Note that you can simply see if $row1 holds a true value, rather than
comparing it to the empty string.
Ronald
