markniew2 has asked for the wisdom of the Perl Monks concerning the following question:

Oh monks, enlighten me. Why is sub AUTOLOAD called for each call to $object->SUPER::method(), even after the initial call to sub AUTOLOAD installs an appropriate sub within *{$AUTOLOAD}? This changed in perl 5.18 and baffles me.

pre 5.18: If you installed your own getter/setter then later calls skipped sub AUTOLOAD.

post 5.18 (to 5.25.5): sub AUTOLOAD is called for every $bar->SUPER::method() invocation, and warns about redefinition. 

#!/usr/bin/perl

use strict;
use warnings;

package Bar;
our @ISA = qw(Foo);

sub data {
    my $self = shift;
    return $self->SUPER::data(@_);
}

package Foo;

our $AUTOLOAD;
sub AUTOLOAD {
    my $self = shift;
    return if $AUTOLOAD =~ /^(.*::)?DESTROY$/;
    
    no strict 'refs';
    my $old = defined *{$AUTOLOAD} ? *{$AUTOLOAD}{CODE}//'' : '';

    my $force_new_code_ref = '';
    *{$AUTOLOAD} = sub { warn "running sub $force_new_code_ref"; };
    # usually a getter/setter, but simplified here.

    my $new = *{$AUTOLOAD}{CODE};
    warn "installed sub AUTOLOAD=$AUTOLOAD new=$new old=$old";

    &$new($self, @_);
}


package main;
my $bar = bless {}, 'Bar';

$bar->data('baz');
$bar->data('bip');
$bar->data('bop');
$bar->data('boo');

[download]

Here are the results for select versions (5.14.4, 5.16.3, 5.18.4, 5.25.5) 

$ perlbrew switch 5.14.4
$ perl ./doit.pl
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x25e5d48) old= at ./
+doit.pl line 29.
running sub  at ./doit.pl line 25.
running sub  at ./doit.pl line 25.
running sub  at ./doit.pl line 25.
running sub  at ./doit.pl line 25.


$ perlbrew switch 5.16.3
$ perl ./doit.pl # v5.16.3
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x1f32970) old= at ./
+doit.pl line 29.
running sub  at ./doit.pl line 25.
running sub  at ./doit.pl line 25.
running sub  at ./doit.pl line 25.
running sub  at ./doit.pl line 25.

$ perlbrew switch 5.18.4
$ perl ./doit.pl
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x16138e8) old= at ./
+doit.pl line 29.
running sub  at ./doit.pl line 25.
Subroutine Bar::SUPER::data redefined at ./doit.pl line 25.
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x1613858) old=CODE(0
+x16138e8) at ./doit.pl line 29.
running sub  at ./doit.pl line 25.
Subroutine Bar::SUPER::data redefined at ./doit.pl line 25.
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x1613768) old=CODE(0
+x1613858) at ./doit.pl line 29.
running sub  at ./doit.pl line 25.
Subroutine Bar::SUPER::data redefined at ./doit.pl line 25.
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x1613888) old=CODE(0
+x1613768) at ./doit.pl line 29.
running sub  at ./doit.pl line 25.


$ perlbrew switch perl-blead
$ perl -v | grep built
This is perl 5, version 25, subversion 5 (v5.25.5 (v5.25.4-61-g483efd0
+)) built for x86_64-linux
$ perl ./doit.pl
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x1aeedc8) old= at ./
+doit.pl line 29.
running sub  at ./doit.pl line 25.
Subroutine Bar::SUPER::data redefined at ./doit.pl line 25.
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x1aeed38) old=CODE(0
+x1aeedc8) at ./doit.pl line 29.
running sub  at ./doit.pl line 25.
Subroutine Bar::SUPER::data redefined at ./doit.pl line 25.
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x1aeec48) old=CODE(0
+x1aeed38) at ./doit.pl line 29.
running sub  at ./doit.pl line 25.
Subroutine Bar::SUPER::data redefined at ./doit.pl line 25.
installed sub AUTOLOAD=Bar::SUPER::data new=CODE(0x1aeed68) old=CODE(0
+x1aeec48) at ./doit.pl line 29.
running sub  at ./doit.pl line 25.

[download]

Adding no warnings 'redefine'; within sub AUTOLOAD bothers me. Creating a sub on every getter/setter invocation seems far from ideal.

I could avoid creating the subs by doing so only if ( ( not defined *{$AUTOLOAD} ) && ( not defined *{$AUTOLOAD}{CODE} ) ), but isn't that exactly what perl itself should be checking before calling sub AUTOLOAD? If a method exists there, why is sub AUTOLOAD being called?

Replies are listed 'Best First'.
Re: AUTOLOAD called on every $object->SUPER::method()
by Anonymous Monk on Sep 02, 2016 at 03:43 UTC
    perl5200delta says
    Perl 5.18 broke autoloading via ->SUPER::foo method calls by looking up AUTOLOAD from the current package rather than the current package's superclass. This has been fixed. perl #120694
[reply]
[reply]

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