[sf-perl] aliases, confusion, and me....

George Hartzell hartzell at alerce.com
Mon Apr 22 16:19:58 PDT 2013

Given the following little bit of code.

    use strict;
    use warnings;
    use Test::More;
        package Poodle;
        sub new { return bless {} => __PACKAGE__; }
        sub this { my $self = shift; $self->{foo} }
        sub that { $_[0]->{foo} }
        sub the_other { $_[0]{foo} }
    my $p = Poodle->new();
    $p->{foo} = 10;
    is( $p->{foo},     10, 'reaching into hash worked' );
    is( $p->this,      10, 'self works' );
    is( $p->that,      10, 'proper dreferencing of alias worked' );
    is( $p->the_other, 10, 'HUH?' );

and looking at the methods "this", "that", and "the other".

I consider "this" to be the "right" way to write a method [or
something like this: "my ($self) = @_;" instead of the shift...]

I often see and sometimes use the technique illustrated in "that",
where one directly accesses the alias in the arguments.  I thought I
understood the implications of it being an aliases to the value being
passed in.

I was stumped as to why "the_other" works.  I figured that it must be
some little subtlety of aliases and argument list and dusty language

Then I used:

    perl -MO=Deparse  ~/tmp/ape.pl

to look at how perl's parsing it.

    sub that {
        use warnings;
        use strict;
    sub the_other {
        use warnings;
        use strict;

While I now understand why "that" and "the_other" work identically, I
now _do not_ understand why perl's handling them the way that it is.

The same phenomena occurs in a class based on a blessed array ref.

Can anyone explain what's going on?



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