I tend to think of global variables as "omnipresent variables". Variables contain data, and should be scoped to the smallest enclosing block where the data is being used. Sometimes, it just makes sense to have data available everywhere. Whether the program is running in debugging mode for instance. Or access to the toplevel window of a program.
Sometimes it saves a lot of typing (and some schools say the smaller the program, the less bugs it contains) to have an omnipresent variable than pass the variable around to almost every subroutine you call.
But if you prefer simple rules and don't want to spend a few braincells wondering whether it makes sense to deviate from a guideline, you can always do:
package main;
my $MW = ...; # Create your main window here.
sub UNIVERSAL::MW {$MW}
After all, subroutine are (global) variables as well - but noone ever yells at you for using them. | [reply]
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some schools say the smaller the program, the less bugs it contains
... like this? ;)
But if you prefer simple rules...
I'm not sure I always prefer simple rules. In case I have to stick to global variables, I'd prefer that this is clearly marked in some way, which is why I'd avoid this implementation of the "singleton". Moreover, this could be no singleton at all, due to the possible presence of other MW methods in other packages.
Flavio
perl -ple'$_=reverse' <<<ti.xittelop@oivalf
Don't fool yourself.
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In the ABOUT package, how to I include the main package.
I am getting the following error when I try this $main::MW.
Can't locate main.pm in @INC (@INC contains: . C:/Perl/lib C:/Perl/sit
+e/lib) atline 8.
I used Data Dumper and the variable is UNDEF.
use main?????
use bpp.pl (the main file name)????
thanks | [reply]
[d/l] |
You don't need to include anything, packages are always there for you! By default, you *are* in package main unless you specify otherwise. An example will be hopefully useful:
poletti@PolettiX:~/sviluppo/perl$ nl mikasue.pl
1 #!/usr/bin/perl
2 use strict;
3 use warnings;
4 { # scope reduction
5 our $MW = "Hello, mikasue!";
6 print "\$MW is '$MW'\n";
7 print "Current package is ", __PACKAGE__, "\n";
8 print "\$main::MW is '$main::MW'\n";
9 } # end of "our" scope
10 package Whatever;
11 no strict; # note: no strict here!
12 print "\$MW is '$MW'\n";
13 print "Current package is ", __PACKAGE__, "\n";
14 print "\$main::MW is '$main::MW'\n";
poletti@PolettiX:~/sviluppo/perl$ perl mikasue.pl
Name "Whatever::MW" used only once: possible typo at mikasue.pl line 1
+2.
$MW is 'Hello, mikasue!'
Current package is main
$main::MW is 'Hello, mikasue!'
Use of uninitialized value in concatenation (.) or string at mikasue.p
+l line 12.$MW is ''
Current package is Whatever
$main::MW is 'Hello, mikasue!'
Note that you have two warnings regarding line 12. The first one is given during the compilation phase, and refers to the misterious "Whatever::MW" variable. This stems from the fact that, when not using strict, every variable you use actually is the corresponding package variable (in the current package). In line 9 we're entering package "Whatever", so there you are. The second warning happens at run time, when you try to print the content of this variable, that is undefined.
As you can see (line 7), by default we're in package main.
Flavio
perl -ple'$_=reverse' <<<ti.xittelop@oivalf
Don't fool yourself.
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