use vars

[jeffa]

Can someone explain what the difference between the two
methods of declaring variable below:

A: my ($rows, $cols);

B: use vars qw($rows $cols);

I am not familiar with using use in the second method.
Thanks.

[autark]

When declaring a variable with 'my', you say that it is lexical, that is, the variable is only visible within the scope that you declared it.

If you on the other hand 'use vars', you say that "in this package I want to use variable $foo without having to write out it's fully qualified name ($main::foo). $foo will not be lexically scoped, but will be a global (a package variable).

In perl 5.6 you may also use the keyword 'our' to declare variables as globals, but with a visibility scope equal to a lexical variable. A global with lexical scope ? That did sound strange :) Writing 'our $foo' inside a block or subrutine will make $foo a global - but you may refer to it as $foo _only_ within the lexical scope. You can still refer to $foo outside the scope, but then you will have to fully qualify it as $main::foo (or $my_package::foo)

Autark

[merlyn]

my creates lexical variables.

use vars allows package variables to be used without colons in the face of use strict.

-- Randal L. Schwartz, Perl hacker

[ybiC]

I've employed both these methods for declaring variables with use strict, but like jeffa, am unclear on the difference(s) between my and use vars.

merlyn's response is, as always, authoritative and to the point. Would a grokful Monk expound a bit, to help an amatuer Perler understand?

Update: Oops - Autark explained very well the lexical vs. global aspect. That's what I get for composing a question and not posting till after finishing a good show on The History Channel. :^)

Now I'm merely confused on the main:foo part. To my knowledge, I've never "fully-qualified" a variable, yet my cobbled-up scripts work anyway. At the risk of asking a dumb question - under what circumstances is it necessary to fully qualify?

[chromatic]

You would fully qualify a 'package' variable when you're in another package. For example, if you use the DBI module, you can access error information for the last call in $DBI::errstr. Presumably, your code is in the main:: package, or another self-defined package. Saying $DBI:: (variable name) tells Perl where to find (variable name), since it's not in the current package.