in reply to (MeowChow) Re: Aliasing Variables
in thread Aliasing Variables

I found Alias, and it works by using blobs. It only creates package variables, not lexicals. That plays havoc with use strict.

But it brings up another question. It implies that local can restore this binding. But Abigail's code on another thread implies that FETCH/STORE is used when localizing a tied variable, as opposed to just changing the binding. Does local work differently on tied variables, or is something else going on?

—John

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(MeowChow - tied local wierdness) Re3: Aliasing Variables
by MeowChow (Vicar) on Jul 22, 2001 at 23:40 UTC
    Look carefully at the sequence and line numbers of the carp output from this:
      
    use strict;
use vars qw($obj);
use Carp qw(carp croak cluck confess);
use Cwd;

sub TIESCALAR { my $var; carp "tie:\t",\$var," = $_[0]\n"; bless \$var
+ => shift;  }
sub FETCH     { carp "fetch:\t$_[0] == ${$_[0]}\n"; ${$_[0]}  }
sub STORE     { carp "store:\t$_[0] =  $_[1]\n"; ${$_[0]} = $_[1]}

tie $obj => 'main';
$obj = 'foo';
{
  my $x = 1;
  my $y = $obj;
  local $obj = 'bar';
}

### OUTPUT ###

tie:    SCALAR(0x8106d78) = main
        main::TIESCALAR('main') called at - line 10
store:  main=SCALAR(0x8106d78) =  foo
        main::STORE('main=SCALAR(0x8106d78)', 'foo') called at - line 
+11
fetch:  main=SCALAR(0x8106d78) == foo
        main::FETCH('main=SCALAR(0x8106d78)') called at - line 14
fetch:  main=SCALAR(0x8106d78) == foo
        main::FETCH('main=SCALAR(0x8106d78)') called at - line 15
store:  main=SCALAR(0x8106d78) =  
        main::STORE('main=SCALAR(0x8106d78)', undef) called at - line 
+15
store:  main=SCALAR(0x8106d78) =  bar
        main::STORE('main=SCALAR(0x8106d78)', 'bar') called at - line 
+15
store:  main=SCALAR(0x8106d78) =  foo
        main::STORE('main=SCALAR(0x8106d78)', 'foo') called at - line 
+13

    [download]
    I'm not sure exactly what you're asking, but I think this may answer your question :-) 
       MeowChow                                   
               s aamecha.s a..a\u$&owag.print
[reply]
[d/l]
      Yes, that matches what this node relies on, that local works by saving the old value and restoring it, exactly like assignments would. In your example, $obj is always pointing to the same SV.

      However, using Alias, e.g. from the module: 

      @DYNAMIC = qw(m n o);
{
 local @DYNAMIC;
 alias DYNAMIC => $tmp, PERM => $tmp;
 $DYNAMIC[2] = 'zzz';
 # prints "abzzzd|abzzzd|abzzzd"
 print @$tmp, "|", @DYNAMIC, "|", @PERM, "\n";
 }
# prints "mno|pqr"
print @DYNAMIC, "|", @PERM, "\n";

      [download]
      The Alias re-binds the symbol @DYNAMIC, essentially the same as *DYNAMIC=$tmp;. @DYNAMIC now aliases @$tmp.

      But look at the local. It will restore the list @DYNAMIC, which means it will un-do this binding operation. If going out of scope does a STORE as you show, essentially @DYNAMIC=@saved;, it will overwrite the contents of @$tmp, as opposed to restoring the action of the *DYNAMIC=$tmp;. Follow me? He did not say local *DYNAMIC.

      If local worked by pointing the symbol to a new SV, then there would be no need to FETCH/STORE, and the tie would not be preserved.

      So there is still more to it than meets the eye.

      —John

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