Re: Magic [Slightly Off-Topic... please point me to documentation]
On Tue, 6 Feb 2001 17:53:17 -0200, Branden wrote:
>It appears you're blessing one reference and returning another... like
>
>sub new {
>my $key;
>my $a = \$key;
>my $b = \$key;
>bless $a;
>return $b;
>}
>
>I think the problem is not with the overloading magic, but with the code
>snippet...
A recent thread on comp.lang.perl.misc discussed how bless() works with
the reference, but alledgedly, it's the underlying thing that gets
blessed, not the reference itself.
my $a = \$x;
my $b = \$x;
bless $a, 'FOO';
print $b;
-->
FOO=SCALAR(0x8a652e4)
It sure looks that they're right. Oh, this is perl 5.6.0.
--
Bart.
Re: Magic [Slightly Off-Topic... please point me to documentation]
Garrett Goebel wrote:
> > > package bar;
> > > @ISA = qw(foo);
> > > sub new { bless \my $key; \$key }
> >
It appears you're blessing one reference and returning another... like
sub new {
my $key;
my $a = \$key;
my $b = \$key;
bless $a;
return $b;
}
I think the problem is not with the overloading magic, but with the code
snippet...
- Branden
Re: Magic [Slightly Off-Topic... please point me to documentation ]
On Tue, Feb 06, 2001 at 01:38:56PM -0600, Garrett Goebel wrote: > I'm sorry... I didn't mean to start an off-topic thread. This is currently being discussed on p5p, so you might want to take it over there. > Is there really no substantial documentation anywhere on magic? Not yet. This looks like something that we can fix. What I'll try and do, although I make no promises, is make sure that either Tim or I [1] finds out all about magic, writes up some documentation and pushes some of it back to perlguts.pod, since it looks like there is definitely a crying need for people to understand this. > I suppose that is why it is called magic, eh? Yup! [1] Tim Jenness has written a section about magic in a book we're writing on Perl and C interaction. However, what we've got at the moment isn't much more than what's in perlguts. -- "Darkly hinting of head hitting desk" -- Megahal (trained on asr), 1998-11-05
RE: Magic [Slightly Off-Topic... please point me to documentation]
From: Branden [mailto:[EMAIL PROTECTED]]
>
> try to define a method in package bar and try to call it
> from $bar, like $bar->foo. Won't work either, you have
> to ${$bar}->foo. Overloading should loose the magic
> in the same sense that the method should not be called.
No, $bar->asString and $baz->asString both work. They produce:
Hello World
> > package bar;
> > @ISA = qw(foo);
> > sub new { bless \my $key; \$key }
>
> You return a reference to the object...
No, it returns an object reference... Try it.
> Try printing $$bar, it will work...
No, it doesn't. $$bar is an undefined scalar.
sub new { my $ref = \ my $key; bless $ref; $ref }
and
sub new { bless \ my $key; \$key; }
Both return an object reference. The former has magic, the latter does not.
I'm sorry... I didn't mean to start an off-topic thread. Unless someone can
find a way to shoe-horn in a discussion of language or internals magic
handling for Perl6? Is there really no substantial documentation anywhere
on magic? I suppose that is why it is called magic, eh?
package foo;
use overload '""' => 'asString';
sub asString { 'Hello World' }
package bar;
@ISA = qw(foo);
sub new { bless \my $key; \$key }
package baz;
@ISA = qw(foo);
sub new { my $ref = \ my $key; bless $ref; $ref }
package main;
my ($bar, $baz);
$bar = bar::->new();
$baz = baz::->new();
print "\$bar$bar\n";
print "\$baz$baz\n";
print "\$bar->asString ", $bar->asString, "\n";
print "\$baz->asString ", $baz->asString, "\n";
print "\$\$bar ", $$bar, "\n";
Results In:
$barbar=SCALAR(0x1bbfc9c)
$bazHello World
$bar->asString Hello World
$baz->asString Hello World
$$bar
Re: Magic [Slightly Off-Topic... please point me to documentation]
Magic [Slightly Off-Topic... please point me to documentation]Garrett Goebel
wrote:
> I was recently bit by overloading magic in the Class::Context
> generic constructor which the following code illustrates. If
> you return "another" reference to a scalar which was blessed,
> instead of the blessed reference, then you loose your magic.
> I'm trying to keep Class::Contract (which I'm maintaining for
> Damian) overload friendly... But Perl magic on my map is
> labelled "there be dragons".
>
Only blessed objects can be overloaded, not references. Only a class can
have overloading behaviour, and a plain reference belongs to no class, so it
should have no overloading behaviour. Contrast this use of \$key with having
[1,2,3]. Do you think this array reference should have any overloading
behaviour? Or else, try to define a method in package bar and try to call it
from $bar, like $bar->foo. Won't work either, you have to ${$bar}->foo.
Overloading should loose the magic in the same sense that the method should
not be called.
> package bar;
> @ISA = qw(foo);
> sub new { bless \my $key; \$key }
You return a reference to the object...
> package main;
> my ($bar, $baz);
> $bar = bar::->new();
> $baz = baz::->new();
>
> print "bar $bar\n";
> print "baz $baz\n";
Try printing $$bar, it will work...
- Branden
Re: Magic [Slightly Off-Topic... please point me to documentation]
At 11:46 AM 2/6/2001 -0600, Garrett Goebel wrote: >From: Dan Sugalski [mailto:[EMAIL PROTECTED]] > > > > No, you attach the magic to a value. Perl just doesn't copy > > magic when it copies data. Whether this is a good thing or > > not is up in the air. (Half the time I want it to, the other > > half I don't...) > >Is there a good discussion of magic, copying magic etc. in the core perl >documentation? Or elsewhere for that matter? Any documentation, pointers, >general tips, etc. would be greatly appreciated. Nope. >I was recently bit by overloading magic in the Class::Context generic >constructor which the following code illustrates. If you return "another" >reference to a scalar which was blessed, instead of the blessed reference, >then you loose your magic. I'm trying to keep Class::Contract (which I'm >maintaining for Damian) overload friendly... But Perl magic on my map is >labelled "there be dragons". Dragons is right. It's a dicey area, and past that most extensions handle magic values flat-out wrong. (Including most of mine, so I've no room to talk...) Dan --"it's like this"--- Dan Sugalski even samurai [EMAIL PROTECTED] have teddy bears and even teddy bears get drunk
Magic [Slightly Off-Topic... please point me to documentation]
From: Dan Sugalski [mailto:[EMAIL PROTECTED]]
>
> No, you attach the magic to a value. Perl just doesn't copy
> magic when it copies data. Whether this is a good thing or
> not is up in the air. (Half the time I want it to, the other
> half I don't...)
Is there a good discussion of magic, copying magic etc. in the core perl
documentation? Or elsewhere for that matter? Any documentation, pointers,
general tips, etc. would be greatly appreciated.
I was recently bit by overloading magic in the Class::Context generic
constructor which the following code illustrates. If you return "another"
reference to a scalar which was blessed, instead of the blessed reference,
then you loose your magic. I'm trying to keep Class::Contract (which I'm
maintaining for Damian) overload friendly... But Perl magic on my map is
labelled "there be dragons".
package foo;
use overload '""' => 'asString';
sub asString { 'Hello World' }
package bar;
@ISA = qw(foo);
sub new { bless \my $key; \$key }
package baz;
@ISA = qw(foo);
sub new { my $ref = \ my $key; bless $ref; $ref }
package main;
my ($bar, $baz);
$bar = bar::->new();
$baz = baz::->new();
print "bar $bar\n";
print "baz $baz\n";
Results In:
bar bar=SCALAR(0x1bbfc9c)
baz Hello World
