Re: tie/bless interactions?
Peter and Jenda, Thank you both for your information and pointers, I appreciated them. I actually ended up doing something slightly different; I wanted to make method calls use identical syntax, whether they were going to the APC::Event or the Tie::DBI, so what I ended up doing was giving the APC::Event a has-a relationship with the Tie::DBI, and then delegating (through AUTOLOAD) calls to the Tie::DBI. It ended up working great! Dave Storrs -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
tie/bless interactions?
I've got a class called APC::Event. The idea is that when you instantiate
one of these objects, you are signalling that a particular event has
happened; ergo, it takes the information you specified, deduces a bunch
more stuff, and logs all of it to a database. Afterwards it sticks around
and provides an interface to that record (or the whole table, actually).
I've just discovered the Tie::DBI modules, and they seem like exactly what
I want here, but I'm wondering if the following is going to get me in
trouble:
sub new {
my ($proto, %args) = @_;
my %self;
my $database_handle =$args{ dbh }
|| DBH(\%default_database_params,
{PrintError = 0, RaiseError = 1}
);
tie %self, Tie::DBI, { db = $database_handle,
CLOBBER = 1, # Allow INSERT and UPDATE,
# but not DELETE
};
my $class =ref $proto# Are we being called as a class or
|| $proto# instance method?
|| 'APC::Event';
bless \%self, $class;
}
I haven't done that much with tie in the past, and I've never done
anything where I tied an object and then blessed it, so I'm not quite sure
what the implications are. Specifically,
1) As I understand it, tie() uses bless() under the hood; is this correct?
2) I want the object to be an APC::Event, because there is more in the
class than just a constructor. Assuming that the answer to question 1 is
yes, what happens when I rebless a reference? I believe it forgets
all about its original class, but I've never done it before (actually,
I've tried very hard to avoid it).
3) If I bless %self out of the Tie::Hash space and into the APC::Event
space, are the Tie::Hash functions going to stop working? (I assume
so.)
4) If the answer to 3 is yes, can I solve this by doing this:
my $rh_self = \%self;
unshift @{$rh_self-ISA}, 'APC::Event';
5) Given the above constructor code, am I likely to have any problems with
circular references?
Thanks in advance,
Dave Storrs
PS If you wanted to rewrite the code snippet in question 4 without taking
a reference, how would you do it? This doesn't seem right:
unshift @self{ISA}, 'APC::Event';
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Re: tie/bless interactions?
From: Dave Storrs [EMAIL PROTECTED]
I've got a class called APC::Event. The idea is that when you
instantiate one of these objects, you are signalling that a particular
event has happened; ergo, it takes the information you specified,
deduces a bunch more stuff, and logs all of it to a database.
Afterwards it sticks around and provides an interface to that record
(or the whole table, actually).
I've just discovered the Tie::DBI modules, and they seem like exactly
what I want here, but I'm wondering if the following is going to get
me in trouble:
sub new {
my ($proto, %args) = @_;
my %self;
my $database_handle =$args{ dbh }
|| DBH(\%default_database_params,
{PrintError = 0, RaiseError = 1}
);
tie %self, Tie::DBI, { db = $database_handle,
CLOBBER = 1, # Allow INSERT and
UPDATE,
# but not DELETE
};
my $class =ref $proto# Are we being called as a class or
|| $proto# instance method?
|| 'APC::Event';
bless \%self, $class;
}
I think this should be OK.
I haven't done that much with tie in the past, and I've never
done
anything where I tied an object and then blessed it, so I'm not quite
sure what the implications are. Specifically,
1) As I understand it, tie() uses bless() under the hood; is this
correct?
Yes, but ... when you tie a variable the
TIESCALAR/TIEARRAY/TIEHASH/TIEHANDLE subroutine
constructs a data structure, creates a reference to that structure,
bless()es the reference, returns the blessed reference and then perl
ties the variable in question with that object.
So it's not the variable, but the hidden reference that's blessed.
2) I want the object to be an APC::Event, because there is more in the
class than just a constructor. Assuming that the answer to question 1
is yes, what happens when I rebless a reference? I believe it
forgets all about its original class, but I've never done it before
(actually, I've tried very hard to avoid it).
Yes, if you rebless a reference it forgets about the original class
and becomes an object of a different class. But this is not what
happens at this time
3) If I bless %self out of the Tie::Hash space and into the
APC::Event space, are the Tie::Hash functions going to stop
working? (I assume so.)
You can't bless a hash, you can only bless a reference.
What you end up with after your constructor is something like :
APC::Event=HASH(0x1a7f148) reference
pointing to
a hash
tied to a
Tie::DBI=HASH(0x486454) reference
pointing to
whatever data Tie::DBI needs to store.
Then
$obj-Foo()
calls the APC::Event::Foo() subroutine and
$val = $obj-{foo}
calls Tie::DBI::FETCH() subroutine with the
Tie::DBI=HASH(0x486454) reference and 'foo' parameters.
Jenda
=== [EMAIL PROTECTED] == http://Jenda.Krynicky.cz ==
There is a reason for living. There must be. I've seen it somewhere.
It's just that in the mess on my table ... and in my brain.
I can't find it.
--- me
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Re: tie/bless interactions?
At 02:19 AM 3/14/02 +0100, Jenda Krynicky wrote:
3) If I bless %self out of the Tie::Hash space and into the
APC::Event space, are the Tie::Hash functions going to stop
working? (I assume so.)
You can't bless a hash, you can only bless a reference.
What you end up with after your constructor is something like :
APC::Event=HASH(0x1a7f148) reference
pointing to
a hash
tied to a
Tie::DBI=HASH(0x486454) reference
pointing to
whatever data Tie::DBI needs to store.
Then
$obj-Foo()
calls the APC::Event::Foo() subroutine and
$val = $obj-{foo}
calls Tie::DBI::FETCH() subroutine with the
Tie::DBI=HASH(0x486454) reference and 'foo' parameters.
Damian Conway's Regexp::Common module does more or less exactly this. I
just finished reading the Perl Conference proceedings on it (yes, in places
I am that far behind) and hope to make a full recovery :-)
--
Peter Scott
Pacific Systems Design Technologies
http://www.perldebugged.com
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