Simple OO Question

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I have this package.

package Sample_Test;

use strict;
use Exporter;
our $VERSION = 1.00;
our @ISA = qw( Exporter );
our @EXPORT = qw();

sub New {
    my $class = shift;
    my $self = {
        user => 'me',
        num => '123',
    };

    bless( $self, $class );
    return $self;
}

sub _Print_Me {
    my ($self) = @_;

    print $self->{user};
}

sub Show_Me {
    my ($self) = @_;

    return _Print_Me();
}

1
[download]

And I have this test script.

#!/usr/bin/perl

use warnings;
use strict;
use Sample_Test;

my $object = Sample_Test->New();

$object->_Print_Me();
$object->Show_Me();
[download]

When I run it, it only gives me the output from $object->Show_Me(). I think it is obvious why it is not working, but I cannot just figure it out.

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Re: Simple OO Question by Corion (Patriarch) on Sep 15, 2010 at 22:20 UTC
That's not what I get: `>perl -w tmp.pl Use of uninitialized value in print at tmp.pl line 25. me` [download] That is because in `->Show_Me`, you're calling `_Print_me` while you should be calling `->_Print_Me` instead: `sub Show_Me { my ($self) = @_; return $self->_Print_Me(); # <-- }` [download]	[reply] [d/l] [select]
Re^2: Simple OO Question by DStaal (Chaplain) on Sep 16, 2010 at 13:02 UTC
As an expansion: `_Print_Me()` is expecting an argument (a reference to a hash), which it puts in `$self`. It then prints the hash element `user` out of that argument. In your version of `Show_Me`, you don't pass it that expected argument. There are two ways to do so in this case: Using standard function notation (`_Print_Me($self)`) or using object notation, which implies the object itself as the first argument. (`$self->_Print_Me()`)	[reply] [d/l] [select]
Re^3: Simple OO Question by dsheroh (Monsignor) on Sep 16, 2010 at 13:21 UTC
As a further expansion: The difference between `_Print_Me($self)` and `$self->_Print_Me` is that `_Print_Me($self)` will always use the current package's `_Print_Me`, while `$self->_Print_Me` will find the appropriate `_Print_Me` for the package (class) that `$self` is blessed into. As a general rule, you'll want to use `_Print_Me($self)` when you want to use one specific implementation, no matter what, and use `$self->_Print_Me` when you want to allow for polymorphism (which should be almost always, since you never know when `$self` might belong to a subclass which has overridden `_Print_Me`).	[reply] [d/l] [select]