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

I must confess that I was writing some code and I omitted the 'sub' declaration in front of AUTOLOAD. Since it half-way works, it was a pain to troubleshoot. But I was surprised that it worked at all without even a warning. I vaguely recall reading about something similar but I can't find the documentation on it. Can anyone say why? Example: 

use strict;
use warnings;

package Foo;
our $AUTOLOAD;

sub new { return bless {}, shift }
sub do_something { print "did something" }

AUTOLOAD {      # note it's missing the 'sub' declaration
        print "message from Foo: undefined method ($AUTOLOAD) called";
}

package main;

my $instance = Foo->new;

$instance->do_something; # prints "did something"
$instance->do_notimplemented; # prints "message from Foo: undefinded m
+ethod (Foo::do_notimplemented) called

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Re: Why does this work? AUTOLOAD sans sub declaration
by converter (Priest) on Nov 10, 2003 at 21:06 UTC

    Since the perlsub POD mentions the AUTOLOAD subroutine in the same sentence as "BEGIN", "CHECK", "INIT", "END", "CLONE" and "DESTROY", I assume it shares some of the same characteristics as the others, the important one in this case being that the sub keyword is implied. When fed your code, B::Deparse prints the following. Notice how the sub keyword is inserted for the BEGIN subs, as well as the AUTOLOAD sub: 

    package Foo;
use warnings;
use strict 'refs';
our $AUTOLOAD;
sub main::BEGIN {
    package main;
    no strict 'refs';
    require strict;
    do {
        'strict'->import
    };
}
sub main::BEGIN {
    package main;
    require warnings;
    do {
        'warnings'->import
    };
}
sub new {
    return bless({}, shift @_);
}
sub do_something {
    print 'did something';
}
sub AUTOLOAD {
    print "message from Foo: undefined method ($AUTOLOAD) called";
}
package main;
my $instance = 'Foo'->new;
$instance->do_something;
$instance->do_notimplemented;

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    converter

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Re: Why does this work? AUTOLOAD sans sub declaration
by hardburn (Abbot) on Nov 10, 2003 at 21:12 UTC

    When I ran your code through B::Deparse, it automagically put a sub keyword before AUTOLOAD. I don't remember this being documented anywhere, though there is plenty of stuff in Perl's documentation that is easy to overlook :)

    ----
    I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
    -- Schemer

    : () { :|:& };:

    Note: All code is untested, unless otherwise stated

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Re: Why does this work? AUTOLOAD sans sub declaration
by duff (Parson) on Nov 10, 2003 at 21:44 UTC

    What works "half-way" about it?

    I was all set to say that this is documented behavior, but then I couldn't find where it was documented. I guess it's just part of the things I've picked up over time that one of the ways that all-caps sub names are special is that the word "sub" is optional.

    Anyone know what POD this is documented in or if it's documented at all?

    -Scott
[reply]
      Not all all-caps functions imply the sub. The tie-related ones don't.

      Makeshifts last the longest.

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Re: Why does this work? AUTOLOAD sans sub declaration
by batkins (Chaplain) on Nov 10, 2003 at 20:50 UTC
    That is indeed odd. Try putting a semicolon on the line that defines do_something.

    Still odd even if that does fix it.

    Are you sure it was a book? Are you sure it wasn't.....nothing?
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      Named subroutine definitions aren't expressions. They don't need semicolons after the block.

[reply]
        Yeah, you're absolutely right. I think I was thinking of prototypes.

        Are you sure it was a book? Are you sure it wasn't.....nothing?
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