[Omaha.pm] Class::Date - change once set

[Jay Hannah]

Forwarding dLux's message.

j


From: "Balázs Szabó (dLux)" <dlux at dlux.hu>
Date: November 11, 2005 3:06:01 AM CST
To: Jay Hannah <jay at jays.net>
Subject: Re: [Omaha.pm] FW: FW: Class::Date - change once set

Hi,

Jay Hannah wrote:

> From: "Balázs Szabó (dLux)" [mailto:dlux at dlux.hu]
>
>> use Class::Date qw(date);
>> $a = date "2000-11-11";
>> $b = date "1970-10-21";
>>
>> print "a: $a, b: $b\n";
>> swap1($a, $b);
>> print "a: $a, b: $b\n";
>>
>> sub swap1 {
>>    $x = $_[0];
>>    $_[0] = $_[1];
>>    $_[1] = $x;
>> }
>
>
> Ahhh, yes. I misread your email the first time. To make sure I know
> what's happening let me walk through it...

Ok.

>
>> sub swap1 {
>
>
> $a is obj in year 2000. $b is obj in year 1970. $_[0] is a ref to $a.
> $_[1] is a ref to $b.

Correct.

>>    $x = $_[0];
>
>
> $x is created, a new obj in year 2000.  (via clone() inside 
> Class::Date)

Not correct.  $_[0] is always points to the same object as $a, $_[1]
always points to the same object as $b; In this case, we have a new
object, $x, which is also points to the same as $a and $_[0];

>>    $_[0] = $_[1];
>
> $a obj is destroyed. A new $a is created, year 1970.  (via clone()
> inside Class::Date)

Not correct. Now $a is points to the original $b, while $x keeps the
reference to $a;

>>    $_[1] = $x;
>> }
>
> $b obj is destroyed. A new $b is created, year 2000.  (via clone()
> inside Class::Date)
>
> Is that right?

Not correct. $b now points to the $x, which kept the reference to the
original $a, and in the previous step, we saw that $a is now pointing to
the original $b;

>
> Thanks,
>
> j
>
So, it does not copy OBJECTS, it just increasing and decreasing
reference counters to objects (since perl is a reference-counting 
language).

Please see the perlobj and perlref (or perlreftut) manual to get what I
had talked about.

Regards,

-- 

Szabó Balázs (dLux)
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