Multiple data members in a perl class - perl

Question

I am new to perl and still learning oop in perl. I usually code in C, C++. It is required to bless an object to notify perl to search for methods in that package first. That's what bless does. And then every function call made with help of -> passes the instance itself as first parameter. Now I have a doubt in writing the constructor for a new object. Normally a constructor would normally look like:
sub new {
my %hash = {};
return bless {%hash}; #will automatically take this package as the class
}
Now I want to have two data members in my class so I can do something like this:
sub new {
my %hash = {};
$hash->{"table_header"} = shift #_; #add element to hash
$hash->{"body_content"} = shift #_;
return bless {%hash}; #will automatically take this package as the class
}
My question is that is this the only possible way. Can't we have multiple data members like in C and C++ and we do have to use strings like "table_header" and "body_content".
EDIT:
In C or C++ we can directly reference the data member(assume its public for now). Here there is one extra reference which has to be made. I wanted to know if there is any way we can have a C like object.
sub new {
my $table_header = shift #_;
my $body_content = shift #_;
#bless somehow
}
Hope this clears some confusion.

Answer 1

There are modules that make OOP in Perl easier. The most important is Moose:
use strict; use warnings;
package SomeObject;
use Moose; # this is now a Moose class
# declare some members. Note that everything is "public"
has table_header => (
is => 'ro', # read-only access
);
has body_content => (
is => 'rw', # read-write access
);
# a "new" method is autogenerated
# some method that uses these fields.
# Note that the members can only be accessed via methods.
# This guards against typos that can't be easily caught with hashes.
sub display {
my ($self) = #_;
my $underline = "=" x (length $self->table_header);
return $self->table_header . "\n" . $underline . "\n\n" . $self->body_content . "\n";
}
package main;
# the "new" takes keyword arguments
my $instance = SomeObject->new(
table_header => "This is a header",
body_content => "Some body content",
);
$instance->body_content("Different content"); # set a member
print $instance->display;
# This is a header
# ================
#
# Different content
If you get to know Moose, you will find an object system that is far more flexible than that in Java or C++, as it takes ideas from Perl6 and the Common Lisp Object System. Of course, this is fairly ugly, but it works well in practice.
Because of the way Perl OOP works, it isn't possible to have the instance members accessible as variables on their own. Well, almost. There is the experimental mop module which does exactly that.
use strict; use warnings;
use mop;
class SomeObject {
# Instance variables start with $!..., and behave like ordinary variables
# If you make them externally accessible with "is ro" or "is rw", then
# appropriate accessor methods are additionally generated.
# a private member with public read-only accessor,
# which has to be initialized in the constructor.
has $!table_header is ro = die 'Please specify a "table_header"!';
# a private member with public read-write accessor,
# which is optional.
has $!body_content is rw = "";
# new is autogenerated, as in Moose
method display() {
# arguments are handled automatically, so we could also do $self->table_header.
my $underline = "=" x (length $!table_header);
return "$!table_header\n$underline\n\n$!body_content\n";
}
}
# as seen in Moose
my $instance = SomeObject->new(
table_header => "This is a header",
body_content => "Some body content",
);
$instance->body_content("Different content"); # set a member, as in Moose
print $instance->display;
# This is a header
# ================
#
# Different content
Although it has pretty syntax, don't use mop right now for serious projects and stick to Moose instead. If Moose is too heavyweight for you, then you might enjoy lighter alternatives like Mouse or Moo (these three object systems are mostly compatible with each other).

Answer 2

You are getting confused between hashes and hash references. You are also forgetting that the first parameter to any method is the object reference or the name of the package. Perl constructors are inherited like any other method, so you must bless the new object into the correct package for polymorphism to work properly. This code is what you intended
sub new {
my $package = shift;
my %self;
$self{table_header} = shift;
$self{body_content} = shift;
bless \%self, $package;
}
I am not clear what you mean by “directly reference the data member”, but if you hoped that you could avoid writing $self everywhere so that every variable was implicitly an element of the hash then you cannot. Perl is far more flexible than most languages, and can use any blessed reference as an object instance. It is most common to use a hash, but occasionally a reference to an array, a scalar, or even a file handle is more appropriate. The cost of this flexibility is specifying exactly when you are referring to a member of the blessed hash. I don't see that it's too great a burden.
You can always write your code more concisely. The method above can be written
sub new {
my $package = shift;
my %self;
#self{qw/ table_header body_content /} = #_;
bless \%self, $package;
}