package symbol table syntax?

John M. Dlugosz has asked for the wisdom of the Perl Monks concerning the following question:

Why does the following:

print  "what? ", $main::{'VERSION'}, "\n";
[download]

indicate that the expression is a CODE(xxxxx) reference. Shouldn't it be a GLOB?

See perlmod under "Symbol Tables": "The value in each entry of the hash is what you are referring to when you use the *name typeglob notation"

Update: I'm thinking that this hash notation only works when such a symbol actually exists, as opposed to glob notation which autovivifies. Meanwhile, it can be "messed up" by assigning to it improperly.

Solution: Normally, assigning a ref to a glob will overwrite the proper slot. If the hash entry doesn't exist, it's not a glob, so assigning to it makes it that type, not a glob!

The docs imply that this hash syntax is equivilent to the * glob syntax. It's not. The glob syntax works for symbols that don't exist (yet), and the hash only shows existing entries and behaves exactly like a standard hash with respect to auto-vivification. It doesn't know it's supposed to be a glob.

Comment on package symbol table syntax? Download Code

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Re: package symbol table syntax? by Ovid (Cardinal) on Nov 05, 2002 at 05:45 UTC
I don't get the results that you are getting, so I suspect that there is some code missing here. However, I can make an educated guess as to what might be related to this problem. Here's some sample code. `#!/usr/local/bin/perl -w use strict; package Foo; $Foo::VERSION = '.01'; package main; $\|++; print Foo->VERSION,$/; $main::VERSION = '.02'; print main->VERSION, $/; print $main::{'VERSION'},$/; print Foo->asdf;` [download] And here's the output: `.01 .02 *main::VERSION Can't locate object method "asdf" via package "Foo" at ./test.pl line +12.` [download] You'll notice that we had no problem calling a `VERSION()` method in the `%Foo::` namespace (and the `%main::` namespace), but we couldn't call `Foo->asdf()`. `VERSION()` is a special method in the `%UNIVERSAL::` namespace which all classes inherit. By calling this as a class method on the `%main::` namespace, we treat `%main::` as a class, thus inheriting `VERSION()`. I suspect that if we see the rest of your code which exhibits this strange behavior, it will somehow be related to this issue. See `perldoc UNIVERSAL` for more details. Cheers, Ovid Join the Perlmonks Setiathome Group. New address of my CGI Course.	[reply] [d/l] [select]
Re: package symbol table syntax? by broquaint (Abbot) on Nov 05, 2002 at 12:43 UTC
... indicate that the expression is a CODE(xxxxx) reference. Shouldn't it be a GLOB? With both 5.6.1 and 5.8.0 I get nothing, unless `VERSION` is defined in some way, in which case I get a stringified version of the glob `shell> perl -e 'print "what? [", $main::{'VERSION'}, "]\n";' what? [] shell> perl5.8.0 -e 'print "what? [", $main::{'VERSION'}, "]\n";' what? [] shell> perl -e '$VERSION=1; \ print "what? [", $main::{'VERSION'}, "]\n";' what? [main::VERSION] shell> perl5.8.0 -e '$VERSION=1; \ print "what? [", $main::{'VERSION'}, "]\n";' what? [main::VERSION]` [download] You would only get something like `GLOB(0x000000)` if you were stringifying a reference to a `GLOB`. e.g `shell> perl -e '$VERSION=1; \ print "what? [", \$main::{'VERSION'}, "]\n";' what? [GLOB(0x806151c)]` [download] And to get something like `CODE(0x000000)` we need to access the `CODE` slot of the glob `shell> perl -e 'sub VERSION {} \ print "what? [", *main::VERSION{CODE}, "]\n";' what? [CODE(0x8107f30)]` [download] HTH `_________ broquaint`	[reply] [d/l] [select]
Re: package symbol table syntax? by chromatic (Archbishop) on Nov 05, 2002 at 05:46 UTC
You'll have to show more code. Autovivification causes it to print undef on my 5.6.1 install.	[reply]


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