perltutorial
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<h3>Object Oriented Perl examples for OO beginners</h3>
I love Perl but its object-oriented facilities are downright screwy.<br>
I was afraid to touch OO with a ten foot pole until I learned<br>
how to use it from a C++ book ( yes, I am multilingual ). So<br>
as a result I could do OO in C++ but I was still scared of it in<br>
Perl.<p>
A week ago or so, I took out the book "Programming The Perl<br>
DBI" from my local library. It had a very simple object-oriented<br>
script that encouraged me to start experiencing Perl OO.<br>
I wrote my first object-oriented module and I'd like to <br>share it with
you. I strongly recommend that you read the OO documentation<br>
that comes with perl ( <i>perltoot, perlobj</i> ) before looking at the example<br>
scripts, but since the examples are only useful for OO novices I will<br>
try to comment the code as best as I can.<p>
<h3>My own Object Oriented Perl module</h3>
<code>
#!/usr/bin/perl
# Number.pm, a number as an object
package Number; # This is the "Class"
sub new # constructor, this method makes an object
# that belongs to class Number
{
my $class = shift; # $_[0] contains the class name
my $number = shift; # $_[1] contains the value of our number
# it is given by the user as an argument
my $self = {}; # the internal structure we'll use to represent
# the data in our class is a hash reference
bless( $self, $class ); # make $self an object of class $class
$self->{num} = $number; # give $self->{num} the supplied value
# $self->{num} is our internal number
return $self; # a constructor always returns an blessed()
# object
}
sub add # add a number to our object's number
{
my $self = shift; # $_[0] now contains the object on which the method
# was called (executed on)
my $add = shift; # number to add to our number
$self->{num} += $add; # add
return $self->{num};
# by returning our new number after each operation we could see
# its value easily, or we could use the dump() method which could
# show us the number without modifying its value.
}
sub subtract # subtract from our number
{
my $self = shift; # our object's internal data structure, as above
my $sub = shift;
$self->{num} -= $sub;
return $self->{num};
}
sub change # assign new value to our number
{
my $self = shift;
my $newnum = shift;
$self->{num} = $newnum;
return self->{num};
}
sub dump # return our number
{
my $self = shift;
return $self->{num};
}
1; # this 1; is neccessary for our class to work
</code>
A method ( all the subroutines in the script ) is a command that
you can execute on an object. It acts on the object's internal data,
which in this case is represented as a hash.
<code>$self = {};</code>
$self is passed to bless() and it must be a reference to
whatever internal data structure we're using, it could be an array
<code>$self = [];</code>
or a scalar reference.
<h3>Using the Object Oriented module</h3>
Having the Number module is fine and dandy but if you cant
use it than...its useless.<br>Here is a little script that uses Number.pm
<code>
#!/usr/bin/perl
# useit.pl using the Number module
use Number;
$obj = Number->new(7); # call the constructor w/a value
# $obj now contains our object from before. remember
# that new() returned it ( in the guise of $self )
print "the number is ",$obj->dump(),"\n";
$obj->add(3); # call the add() method on our object
print "now the number is ",$obj->dump(),"\n";
$num = $obj->subtract(5);
print "now the number is $num\n";
# remember that subtract() returns the current value of our number
# so that $num has that value assigned to it.
$obj->change(999999);
print "number changed to ",$obj->dump(),"\n";
# NOTE: if you include the object method inside the double
# quoted string in print() then it wouldnt work, it'll return the
# memory address of the object and "->dump()" appended
# to it...
exit();
</code>
This script works on the assumption that Number.pm ( the file<br>
containing our class, our module ) is either in the perl module<br>
directory or is in the same directory as itself. To load a module<br>
from somewhere else take a look at the @INC array in <i>perlvar</i>.<br><p>
The output of the script that uses Number.pm is this:<br>
the number is 7<br>
now the number is 10<br>
now the number is 5<br>
number changed to 999999<p>
To see this for yourself just type "perl" at the prompt, paste the code<br>
for Number.pm and then the code for useit.pl right after it onto the same<br>
screen ( this is called interactive-mode ). When your'e done just press the<br>
key combination Control-D and the code will execute. An interesting thing<br>
to remember is that Number.pm doesn't always need its own file, you<br>
can have Number.pm and useit.pl in the same file provided that you<br>
write the line "package Number;".<p>
Whew, that was fairly long. If you dont understand what my<br>
scripts do dont despair, copy them and run them, or read some<br>
OO Perl tutorials. If youre completely stumped then try tackling<br>
OO in C++ since it has a nicer interface to object-orientation<br>
than Perl. Perl is still the greatest though...<p>
- Jacob F<p>
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