Autoloading and anonymous subs

[haoess]

perlsub says:

if an "AUTOLOAD" subroutine is defined in the package or packages used to locate the original subroutine, then that "AUTOLOAD" subroutine is called with the arguments that would have been passed to the original subroutine.

package Foo;
sub AUTOLOAD { __PACKAGE__ }

package main;
sub AUTOLOAD { __PACKAGE__ }

# should print Foo
print $Foo::foo->(); # main

print Foo::foo(); # Foo

Does $Foo::foo->() not mean locate the sub in the package Foo?

-- Frank

[Joost]

$Foo::foo->() calls the subroutine reference in $Foo::foo. (i.e, $foo in the Foo package). Assuming $Foo::foo is undefined and you're not running under strict, that would probably refer to the subroutine "" in the calling package (I'm guessing here, since I tend not to run strict-less)

If you want to call Foo::foo() as a class method of Foo, do:

Foo->foo();

[ysth]

sub AUTOLOAD {"AUTOLOAD"}
*{""} = sub {"empty"};
print $nosuch->();

confirms your suspicion.

[shmem]

Does $Foo::foo->() not mean locate the sub in the package Foo?

No. $Foo::foo is a symbol living in the symbol table Foo, that's granted; but its contents must not neccessarily refer to package Foo. So the normal method lookup rules apply. Which means that AUTOLOAD in the package main is found first.

update: If the content of $Foo::foo is a sub reference in the Foo namespace, the Foo AUTOLOAD is found, eg

$Foo::foo = \&Foo::bar;
print $Foo::foo->(); # Foo

[haoess]

And it's always the main package:

package Foo;
sub AUTOLOAD { __PACKAGE__ }
print $foo->();

package main;
sub AUTOLOAD { __PACKAGE__ }
__END__
main

I just can't find the documentation for this behaviour.

-- Frank

[shmem]

You are calling an undefined coderef. This coderef (which isn't, being just nothing, or not existing) has no association with any package, albeit the variable holding "that nothingness" (if nothing can be something) has. Now, how should perl resolve that sensibly? There's no package, there's no code, there's no sub. Hence the default package (main), and as last resort, AUTOLOAD.

The documentation is implicit in perlsub, perlref, ...

[rir]

How do you take the address of a sub that needs to be AUTOLOADed before it is AUTOLOADed? Hmmm?

Be well,
rir

[shmem]

*shrug* I just do. It's a magic called autovivification :-P
You can take a reference to a sub that doesn't exist. It's much like forward declarations of subs:

package Foo;

sub bar; # we'll define that later

\&bar;   # same effect

In fact, that's how AutoLoader works - it loads a modules autosplit.ix file which contain forward declarations of subs to be loaded. The CODE slot of the typeglob is allocated, but it's empty. That's why calling such a sub AutoLoader's AUTOLOAD is called, which requires the sub's source file on the fly.