(MeowChow - tied local wierdness) Re3: Aliasing Variables

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

Comment on (MeowChow - tied local wierdness) Re3: Aliasing Variables Download Code

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Re: (MeowChow - tied local wierdness) Re3: Aliasing Variables by John M. Dlugosz (Monsignor) on Jul 23, 2001 at 02:50 UTC
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	[reply] [d/l] [select]

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Re: (MeowChow - tied local wierdness) Re3: Aliasing Variables
by John M. Dlugosz (Monsignor) on Jul 23, 2001 at 02:50 UTC

this node

value

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]

*DYNAMIC=$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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