How do I inherit subroutines in Perl with 'use base'? - perl

Question

How do I apply 'use base' in Perl to inherit subs from some base module?
I'm used to C++ inheritance mechanics, and all the sites I googled for this caused more confusion then help. I want to do something like the following:
#! /usr/bin/perl
#The base class to inherit from
use strict;
use warnings;
package 'TestBase';
#-------------------------------
sub tbSub
{
my ($self, $parm) = #_;
print "\nTestBase: $parm\n";
}
1;
.
#! /usr/bin/perl
#The descendent class
use strict;
use warnings;
use base qw(TestBase);
sub main;
sub mySub;
#-------------------------------
#Entry point...
main();
#---code------------------------
sub main
{
mySub(1);
tbSub(2);
mySub(3);
}
#-------------------------------
sub mySub
{
my $parm = shift;
print "\nTester: $parm\n";
}
Perl complains/cannot find tbSub.

Answer 1

The C++ mechnics aren't much different than the Perl mechanics: To use inheritance, you need two classes: the base class and the inheriting class. But you don't have any descendent class.
You are also lacking a constructor. Unlike C++, Perl will not provide a default constructor for you.
Your base class contains a bad syntax error, so I guess you didn't try the code before posting.
Finally, as tsee already observed, you will have to let Perl know whether you want a function call or a method call.
What you really want would look something like this:
my $foo = TestDescendent->new();
$foo->main();
package TestBase;
sub new {
my $class = shift;
return bless {}, $class;
}
sub tbSub
{
my ($self, $parm) = #_;
print "\nTestBase: $parm\n";
}
package TestDescendent;
use base 'TestBase';
sub main {
my $self = shift;
$self->mySub( 1 );
$self->tbSub( 2 );
$self->mySub( 3 );
}
sub mySub
{
my $self = shift;
my $parm = shift;
print "\nTester: $parm\n";
}
1;

Answer 2

You should have a look at using Moose which is a postmodern object system for Perl5. You will probably find it a lot easier to grasp than using standard Perl OO semantics... especially when coming from another OO language.
Here's a Moose version of your question....
package TestBase;
use Moose;
sub tbSub {
my ($self, $parm) = #_;
print "\nTestBase: $parm\n";
}
package TestDescendent;
use Moose;
extends 'TestBase';
sub main {
my $self = shift;
$self->mySub( 1 );
$self->tbSub( 2 );
$self->mySub( 3 );
}
sub mySub {
my ($self, $parm) = #_;
print "\nTester: $parm\n";
}
package main;
my $foo = TestDescendent->new();
$foo->main
The differences are....
Constructor automatically created for you &
Inheritance defined by "extends" command instead of "use base".
So this example only covers the tip of the Moose iceberg ;-)

Answer 3

As a sidenote, there is little good reason to use base rather than the newer use parent.

Answer 4

It seems to me, you are mixing up two things here: Object-Oriented and Procedural Perl. Perl OO is kind of "different" (as in not mainstream but workable).
Your TestBase.pm module seems to expect to be run as a Perl object (Perl oo-style), but your Perl script wants to access it as "normal" module. Perl doesn't work the way C++ does (as you realised) so you would have to construct your code differently. See Damian Conway's books for explanations (and smarter code than mine below).
Procedural:
#! /usr/bin/perl
#The module to inherit from
package TestBase;
use strict;
use warnings;
use Exporter ();
our #ISA = qw (Exporter);
our #EXPORT = qw (tbSub);
#-------------------------------
sub tbSub
{
my ($parm) = #_;
print "\nTestBase: $parm\n";
}
1;
.
#! /usr/bin/perl
#The descendent class
use strict;
use warnings;
use TestBase;
sub main;
sub mySub;
#-------------------------------
#Entry point...
main();
#---code------------------------
sub main
{
mySub(1);
tbSub(2);
mySub(3);
}
#-------------------------------
sub mySub
{
my $parm = shift;
print "\nTester: $parm\n";
}
Perl OO
#! /usr/bin/perl
#The base class to inherit from
package TestBase;
use strict;
use warnings;
#-------------------------------
sub new { my $s={ };
return bless $s;
}
sub tbSub
{
my ($self,$parm) = #_;
print "\nTestBase: $parm\n";
}
1;
.
#! /usr/bin/perl
#The descendent class
use strict;
use warnings;
use TestBase;
sub main;
sub mySub;
#-------------------------------
#Entry point...
main();
#---code------------------------
sub main
{
my $tb = TestBase->new();
mySub(1);
$tb->tbSub(2);
mySub(3);
}
#-------------------------------
sub mySub
{
my $parm = shift;
print "\nTester: $parm\n";
}

Answer 5

Perl's inheritance inherits methods, not functions. That means you will have to call
main->tbSub(2);
However, what you really want is to inherit the method into a proper class:
package Derived;
use base "TestBase";
package main;
Derived->somemethod("foo");
Calling methods in the current package as functions won't pass in the $self or "this" object nor the class name magically. Internally,
Class->somemethod("foo")
essentially ends up being called as
Class::somemethod("Class", "foo")
internally. Of course, this assumes Class has a subroutine/method named "somemethod". If not, the superclasses of Class will be checked and if those don't have a method "somemethod" either, you'll get a fatal error. (Same logic applies for $obj->method("foo").)

Answer 6

OO syntax uses the -> operator to separate the message and arguments from the receiver of the message. A short illustration below.
You->do_something( #params );
OR
$you->do_something( #params );
package A;
sub do_neat_thing {
my ( $class_or_instance, #args ) = #_;
my $class = ref( $class_or_instance );
if ( $class ) {
say "Instance of '$class' does a neat thing.";
}
else {
say "$class_or_instance does a neat thing.";
}
}
...
package main;
A->do_neat_thing(); # A does a neat thing.
my $a_obj = A->new();
$a_obj->do_neat_thing(); # Instance of 'A' does a neat thing.