Re^2: Grabbing Variable Names

[mab]

in reply to Re: Grabbing Variable Names
in thread Grabbing Variable Names

Hi,

I'm very interested in your comments about determining if a function has been forward declared but not yet loaded, e.g., when using AutoLoader. For some reason, though, I can't get this to work. Here's my test case:

sub hello;
*fn = $::{'hello'};
print "declared\n" if *fn{CODE};
print "not defined\n" if ! defined &fn;
print "exists\n" if exists &fn;
print "glob exists\n" if exists $::{'hello'};

For me, this test correctly prints "not defined" and "glob exists" but shouldn't it also print "declared" and "exists?"

Thanks for any insight you can give me on this.

Best,
Mike

[ihb]

Long story short is that a forward declaration doesn't create a glob. You can try this by doing

sub foo;
print ref \$::{foo}; # "SCALAR"

sub bar { 1 };
print ref \$::{bar}; # "GLOB"

sub baz;
print ref \$::{baz}; # "GLOB"
$baz = $baz;         # Just to mention the symbol "baz".

As it turns out if you inspect this a bit further the prototype of the subroutine (or "-1" if no prototype) is stored instead of a glob if no glob has been created already. When a symbol with the same name as the forward declaration a glob is created. So when you do *fn = $::{hello} you actually assign -1 to *fn making *fn aliased to *{-1}. (*{something} is just the "safe" way to write *something for symbols that aren't generally recognized as symbols by Perl and can be used with any sigil, like &{subname}.)

sub foo;
*{-1} = sub { 'oh' };     # Create a sub called "-1".
print &{-1}();            # "oh"
*bar = $::{foo};          # Really?
print defined &bar;       # "1" Huh? So now &bar is defined
                          # even though &foo isn't?
print bar();              # "oh"

#$foo;                    # But all this would change if you
                          # uncommented "$foo;".

Sneaky, eh?

ihb