Re: Re: Trying to learn about References

[vladb]

Certainly I agree with what you said. Thanks for your comments ;-). Probably, what I _really_ meant is doing something like

my ($in) = @_;

will initialize a new variable $in with the contents of the first argument.. and if it's other than a reference the cost of doing so may be high if the data to be thus 'copied' is substantial.

I've done a bit more playing with Devel::Peek module which allows one to look inside the Perl variables (look up their guts sort of ;-):

#!/usr/local/bin/perl

use Devel::Peek 'Dump';
sub foo {
    # this will create a new variable $s and copy first argument's con
+tents right?
    my ($s) = @_;
    print "Dump inside trim():\n";
    print "\$_[0] (this one is infact a 'synonym' to the actual variab
+le; not it's copy :-) :\n";
    Dump($_[0]);print"\n";
    print "\$s (this is a copy of the 1-st argument:\n";
    Dump($s);print"\n";
}

sub bar(\$) {
    # this will create a new variable $s and copy first argument's con
+tents as well,
    # however, it's much faster here compred to foo() since in this ca
+se only
    # a reference to the actual variable is copied, not the actual var
+iable's
    # contents.
    my ($s) = @_;
    print "Dump inside bar(\\\$):\n";
    print "\$_[0]:\n";
    Dump($_[0]);print"\n";
    print "\$s (this is a copy of the 1-st argument, which is nothing 
+more but a reference..):\n";
    Dump($s);print"\n";
}

$s = "some text here ";

print "Dump before:\n";
Dump($s);print"\n";

foo($s);
bar($s);

print "Dump after:\n";
Dump($s);print"\n";

And here's the output produced by this script:

Dump before:
SV = PV(0xf0c44) at 0xfe99c
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0xf7560 "some text here "\0
  CUR = 15
  LEN = 16

Dump inside trim():
$_[0] (this one is infact a 'synonym' to the actual variable; not it's
+ copy :-) :
SV = PV(0xf0c44) at 0xfe99c
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0xf7560 "some text here "\0
  CUR = 15
  LEN = 16

$s (this is a copy of the 1-st argument:
SV = PV(0xf0c98) at 0xfe9c0
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0xf7540 "some text here "\0
  CUR = 15
  LEN = 16

Dump inside bar(\$):
$_[0]:
SV = RV(0x11581c) at 0xf096c
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0xfe99c
  SV = PV(0xf0c44) at 0xfe99c
    REFCNT = 3
    FLAGS = (POK,pPOK)
    PV = 0xf7560 "some text here "\0
    CUR = 15
    LEN = 16

$s (this is a copy of the 1-st argument, which is nothing more but a r
+eference..):
SV = RV(0x115828) at 0x1164b0
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,ROK)
  RV = 0xfe99c
  SV = PV(0xf0c44) at 0xfe99c
    REFCNT = 3
    FLAGS = (POK,pPOK)
    PV = 0xf7560 "some text here "\0
    CUR = 15
    LEN = 16

Dump after:
SV = PV(0xf0c44) at 0xfe99c
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0xf7560 "some text here "\0
  CUR = 15
  LEN = 16

Certainly, the difference is subtle. And, also, '$_[0]' itself is not a copy of the source variable, but rather it's 'synonym' or reference (c-like). However, most of the time it's much easier to simply pass around references just in case, at least that's how me thinks ;-)...

"There is no system but GNU, and Linux is one of its kernels." -- Confession of Faith