Packages
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Default Scope
When a variable or file handle comes into existence or a user function is
defined, the default scope of these entities is bound to a package
called "main". For example, these variables in
this short program are all in the "main" package:
/usr/local/bin/perl -w
opendir (DIR, '.') || die "Can't open current dir: $!\n";
@Files = readdir (DIR);
closedir (DIR);
foreach (@Files)
{
$Size = (stat ($_))[7];
print "$_ has size $Size blocks\n";
}
Why Packages?
Packages can help avoid name collisions in Perl programs. A perfect
place to use them is in user libraries, such as:
<![CDATA[
package UnixTools;
$Version = '1.32'; # a package variable
sub Dirname
{
my $Path = shift;
my ($Name, @Temp);
@Temp = split (/\\|\//, $Path); # Split on forward or backslash
pop @Temp; # Get rid of last element
$Name = join ("/", @Temp);
return $Name;
}
# other functions...
1;
]]>
Accessing Package Stuff
Using the previous example and assuming the code exists in a file
called UnixTools.pm, we can access package variables and
functions from our "main" program this way:
<![CDATA[
/usr/local/bin/perl -w
use UnixTools; # pulls in definitions from that file
$dummy_var = 10; # to be used later
# access a package variable
print "UnixTools version = $UnixTools::Version\n";
# access a package function
print "Root home path = ", &UnixTools::Dirname ($ENV{HOME}), "\n";
]]>
Accessing Other Stuff From a Package
Accessing a variable or function that is defined in another package,
from a package, needs to be scoped. If a package name is not used,
again, the "main" package is the default. So
our UnixTools.pm file could have accessed
$dummy_var defined in "main" by
doing:
<![CDATA[
package UnixTools;
# other stuff
sub IncDummyMainVar
{
$main::dummy_var++;
}
]]>
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Last Modified: $Date: 1997/05/02 07:17:46 $