Re: Localization method of $1

[Ilya Zakharevich <ilya@math.ohio-state.edu>]

On Sun, Jan 16, 2000 at 12:20:25PM -0800, Peter Scott wrote:

[Back to p5p.]

> $x = "foo";
> print "Initial: ", pref $x;
> {
>    local $x = "bar";
>    print "In block: ", pref $x;
> }
> print "After block: ", pref $x;
> 
> Initial: SCALAR(0x80f8db8) = foo
> In block: SCALAR(0x80eb66c) = bar
> After block: SCALAR(0x80f8db8) = foo
> 
> The 'local $x' has a new address; I can still get at the original $x inside 
> the block if I make a reference to it first.

This is an implementation detail - which in principle might be
considered to be a bug any time.

> But that's not what's happening with $1:

Correct.  Saving value of $1 is done via a different mechanism. $1 is
kinda-tied, and localization is done by kinda-changing-the-FETCH-data:

  { package backref1; our $dollar1 = \undef; sub FETCH { $$dollar1 } }

Localization is done by kinda-doing

  local $dollar1 = \$$dollar1;

on the entry into a block (only this \$$dollar1 is kinda-precomputed
at compile time, thus there is no runtime penalty).

The real implementation is not via tie(), but via a different type of
magic.  And access is not via $$dollar1, but via calculations over
non-SV storage, so it cannot be completely expressed via Perl syntax.
But I hope you got the feeling...

Ilya

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Follow-Ups from:

Peter Scott <Peter@PSDT.com>

References to:

Peter Scott <Peter@PSDT.com>
Ilya Zakharevich <ilya@math.ohio-state.edu>

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