Dan Muey wrote:
>
> Howdy,
>
> I read in an earlier email that some people put _ in front of
> variables/function/etc to show that they are "important and should be
> handled by the program only" and it said that some "modules enforce this"
>
> I guess what I'm getting at is:
>
> If my module exports variables can I make it so they can't be changed?
>
> IE
>
> use MyStuff qw($_joe $_mama); # now they should have $_joe and $_mama
> exported from the module.
>
> print "$_joe $_mama\n"; # ok
> my $joe = $_joe; # ok
> $joe =~ s/\W//g; # ok
> $_joe = "new value"; # bad - not allowed - maybe give warning?
> $_mama =~ s/\W//g; # bad - not allowed - maybe give warning?
>
> If so hwo do I do it or what module protects its variables in this way so
> I can look at it's code for an example or documentation???
a lot of people have suggested using closures which is nice. another appoach
is to use tie. simple example:
package ConVar;
sub TIESCALAR{
my $class = shift;
my $value = shift;
return bless \$value => $class;
}
sub FETCH{ ${$_[0]} }
#--
#-- variable that can't be changed
#--
sub STORE{ warn("modification is not allowed\n") }
1;
__END__
package MyModule;
use Exporter;
use ConVar;
our @ISA=qw(Exporter);
our @EXPORT_OK = qw($var1 $var2); #-- export read only
#--
#-- your module implementing the read only variable
#--
tie our $var1,'ConVar',5;
tie our $var2,'ConVar',6;
1;
__END__
#!/usr/bin/perl -w
use strict;
use MyModule qw($var1 $var2);
#--
#-- testing the read only variable
#--
$var1 = 7;
$var2 = 8;
print "$var1\n";
print "$var2\n";
__END__
prints:
modification is not allowed
modification is not allowed
5
6
notes:
* the code is, generally speaking and imo, cleaner.
* user notice no differences (except when they start benchmark their script)
whether $var1 has magic behind it or not.
* you can tie anything (array, hash ref, or another object) behind the
simple scalar interface and it still looks like a simple variable to the
users of your module.
* it's slower than the other method. probably 10 times
* it's not very secure. for example:
my $g = tied $var1;
$$g = 10;
print "$var1\n"; #-- $var1 is now 10.
other tricks and readings can be found at:
perldoc -f tie
perldoc perltie
david
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