R. Joseph Newton wrote:
> "...
> Unlike 'my', however, it is a permanent
> package variable and its value will be retained across calls to a
> subroutine ..." --Rob
>
> Hi Rob,
>
> So does this make it equivalent to a C-style static variable?
Sort of, but beware that any occurrence of 'our $var' in the same
package is the same variable. It's more like Fortran COMMON blocks
if that means anything to you?
> I've been trying to test it, and it seems to be impossible to even
> declare it using strict.
The main point of 'our' is that it lets you use package variable with
'use strict'. Any variable used without first declaring it as 'my' is
a package variable which won't go away. 'use strict' stops you
from using variables without first declaring them, and 'our' lets you
declare them after all.
> When I try to declare a variable as local,
> I get an error telling me that the global symbol needs an explicit
> package name.
That's 'use strict' helping you out! Forget about 'local' for now. All
it does is create a temporary copy of an existing global variable
for the duration of the current scope. The variable has to be
there first or 'use strict' will complain.
> When I try to modify an existing variable, I get an
> error saying that I can't modify a lexical variable.
>
You can't apply 'local' to anything but a global variable. A 'my'
variable is a local variable anyway, so there's no point in making
a temporary local copy of it.
>
> Without strict, I have yet to find any configuration that would
> actually retain value between calls to a function.
>
Easy. Try this:
#!perl
use strict;
use warnings;
sub p {
our $val;
print ++$val, "\n";
}
p;p;p;p;p;p;
__END__
>
> Can you show any examples of local at work--preferably using strict?
>
Like I said, forget about it. I've never used it except for temporarily
changing the value of a Perl predefined variable. For instance, enable
'slurp' mode like this:
my $buffer = do { local $/; <FH> };
you can put
local $var = 'value'
but without the assignment it leaves the variable as 'undef'. The new
value only exists until the end of the scope (i.e. for the duration
of the <FH>) after which Perl puts the old value back for you.
Don't use local apart from for this sort of thing.
>
> I do have a little demo for the our and my scoping, about the way I
> would use them. I actually couldn't detect any difference when
> declared in the global scope, so I take the our declaration as a
> comment, to remind me that I've done something dumb by declaring in
> the global context. Essentially, I would say that visibility
> protection comes down to the location of the declaration.
>
> #!/usr/bin/perl -w
>
> #use strict;
>
> our $FirstName = "Robert";
That's global variable $main::FirstName, but since you've
declared it at file level it's in scope all the way through.
>
> FeedSub();
> sub FeedSub {
> $MiddleName = "Joseph";
$MiddleName is undeclared, so 'use strict' will barf on it.
> my $LastName = "Newton";
A local variable. It will be reallocated and reassigned here
every time the sub is called.
> for (1...4) {
> TestIt($LastName, $MiddleName);
> print "$LastName\n";
> print "$FirstName\n";
> print "$MiddleName\n";
> }
> }
>
> sub TestIt {
> my ($LastName, $MiddleName) = @_;
These two parameters passed by value from FeedSub() and
copied into local (non-persistent) variables.
> print "$LastName\n";
> $MiddleName .= '_';
> $FirstName .= '_';
This is the only package variable that you're changing. It will
grow by one underscore per call if you look at your output
carefully.
> print "$FirstName\n";
> print "$MiddleName\n";
> print "$LastName\n";
> }
I hope this helps. Cheers Joseph,
Rob
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