Passing object from subroutine problem
in [Perl]
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From: tim on 27 Sep 2006 08:08 Hello I'm new here, so, hi! I've been having the following problem. I have a module which I am creating to manage sessions for my website. I have the following sub in it: #takes an active db handle, returns a new session object. sub newSession { my $dbh = @_[1]; my %session; tie %session, 'Apache::Session::MySQL', undef, { Handle => $dbh, LockHandle => $dbh}; print "Session session ref = ". \%session . "\n"; return \%session; } In the calling routine, I have the following code: my $sessionref = SessionManager->newSession($dbh); my %session = %$sessionref; tied(%session)->delete; The first two lines seem to work ok, but the 3rd gives this error: Can't call method "delete" on an undefined value at ./tester line 31. Bizarely, it seems I can access the object in other ways from the same context, as the following lines work as expected: print "session id = $session{_session_id}\n"; print "testing session ref = " . $sessionref . "\n"; (i.e. session id is printed) I'd really appreciate any comments or suggestions on why the above may be occuring. Many thanks Tim.
From: Paul Lalli on 27 Sep 2006 08:18 tim wrote: > I'm new here, so, hi! Welcome! Please make sure you read the Posting Guidelines for this group (posted here twice a week) to learn valuable tips to maximize the potential usefulness of posting here... > I've been having the following problem. I have a module which I am > creating to manage sessions for my website. I have the following sub in > it: > > #takes an active db handle, returns a new session object. > sub newSession { > my $dbh = @_[1]; Always always always use warnings when devleoping Perl code. This should have told you: Scalar value @_[1] better written as $_[1] > my %session; > tie %session, 'Apache::Session::MySQL', undef, > { Handle => $dbh, > LockHandle => $dbh}; > print "Session session ref = ". \%session . "\n"; > return \%session; > } > > In the calling routine, I have the following code: > > my $sessionref = SessionManager->newSession($dbh); > my %session = %$sessionref; %session is a brand new hash, simply populated with the values that are currently in the variable %$sessionref. It is in all other ways unrelated to $sessionref. It is not tied to any object of the Apache::Session::MySQL class. > tied(%session)->delete; > > The first two lines seem to work ok, but the 3rd gives this error: > > Can't call method "delete" on an undefined value at ./tester line 31. Because %session is not a tied variable, tied() returns undef. > > Bizarely, it seems I can access the object in other ways from the same > context, as the following lines work as expected: > > print "session id = $session{_session_id}\n"; _session_id is one of the keys you copied over when you created the new hash, so yes, that works. > print "testing session ref = " . $sessionref . "\n"; > > (i.e. session id is printed) $sessionref is a reference to the original tied variable, so yes, that works. > I'd really appreciate any comments or suggestions on why the above may > be occuring. Copying a hash does not copy the tied nature of the variable. Paul Lalli
From: tim on 27 Sep 2006 08:43 Thanks Paul Whoops- didn't notice that warnings were off. I'll make sure they're on before posting next time!! Tim Paul Lalli wrote: > tim wrote: > > > I'm new here, so, hi! > > Welcome! Please make sure you read the Posting Guidelines for this > group (posted here twice a week) to learn valuable tips to maximize the > potential usefulness of posting here... > > > I've been having the following problem. I have a module which I am > > creating to manage sessions for my website. I have the following sub in > > it: > > > > #takes an active db handle, returns a new session object. > > sub newSession { > > my $dbh = @_[1]; > > Always always always use warnings when devleoping Perl code. This > should have told you: > Scalar value @_[1] better written as $_[1] > > > my %session; > > tie %session, 'Apache::Session::MySQL', undef, > > { Handle => $dbh, > > LockHandle => $dbh}; > > print "Session session ref = ". \%session . "\n"; > > return \%session; > > } > > > > In the calling routine, I have the following code: > > > > my $sessionref = SessionManager->newSession($dbh); > > my %session = %$sessionref; > > %session is a brand new hash, simply populated with the values that are > currently in the variable %$sessionref. It is in all other ways > unrelated to $sessionref. It is not tied to any object of the > Apache::Session::MySQL class. > > > tied(%session)->delete; > > > > The first two lines seem to work ok, but the 3rd gives this error: > > > > Can't call method "delete" on an undefined value at ./tester line 31. > > Because %session is not a tied variable, tied() returns undef. > > > > > Bizarely, it seems I can access the object in other ways from the same > > context, as the following lines work as expected: > > > > print "session id = $session{_session_id}\n"; > > _session_id is one of the keys you copied over when you created the new > hash, so yes, that works. > > > print "testing session ref = " . $sessionref . "\n"; > > > > (i.e. session id is printed) > > $sessionref is a reference to the original tied variable, so yes, that > works. > > > I'd really appreciate any comments or suggestions on why the above may > > be occuring. > > Copying a hash does not copy the tied nature of the variable. > > Paul Lalli
From: Mumia W. (reading news) on 27 Sep 2006 08:47 On 09/27/2006 07:08 AM, tim wrote: > Hello > > I'm new here, so, hi! > > I've been having the following problem. I have a module which I am > creating to manage sessions for my website. I have the following sub in > it: > > #takes an active db handle, returns a new session object. > sub newSession { > my $dbh = @_[1]; > my %session; > tie %session, 'Apache::Session::MySQL', undef, > { Handle => $dbh, > LockHandle => $dbh}; > print "Session session ref = ". \%session . "\n"; > return \%session; > } > > In the calling routine, I have the following code: > > [29:] my $sessionref = SessionManager->newSession($dbh); > [30:] my %session = %$sessionref; > [31:] tied(%session)->delete; > The hash is no longer tied to 'Apache::Session.' Line 30 makes a /copy/ of the tied hash returned by newSession(). However, the tying does not survive the copying, so $sessionref is still tied, but %session is not, so the "tied" function returns undef for %session. > The first two lines seem to work ok, but the 3rd gives this error: > > Can't call method "delete" on an undefined value at ./tester line 31. > It's the same as saying that this is invalid: undef->delete(); > Bizarely, it seems I can access the object in other ways from the same > context, as the following lines work as expected: > > print "session id = $session{_session_id}\n"; > print "testing session ref = " . $sessionref . "\n"; > > (i.e. session id is printed) > Try this: tied(%$sessionref)->delete(); > I'd really appreciate any comments or suggestions on why the above may > be occuring. > > Many thanks > > Tim. > You're welcome. -- paduille.4058.mumia.w(a)earthlink.net
From: Mirco Wahab on 27 Sep 2006 08:51
Thus spoke tim (on 2006-09-27 14:08): > I'm new here, so, hi! Great! > #takes an active db handle, returns a new session object. > sub newSession { > my $dbh = @_[1]; Shouldn't the @_[1], which is a array slice containing the *second* argument of the caller, read: $_[0] ... ? > my $sessionref = SessionManager->newSession($dbh); > my %session = %$sessionref; > tied(%session)->delete; Here you drop a single argument to your function ... (The return will be a ref to empty hash or sth.) Regards Mirco |