Constants and Accessors in Perl OOP

Perl Object Oriented Programming – Part 8

Perl Course

Foreword: In this part of the series, I explain how to code constants and I also explain the recommended way to access attributes.

By: Chrysanthus Date Published: 26 Sep 2015

Introduction

This is part 8 of my series, Perl Object Oriented Programming. In this part of the series, I explain how to code constants and I also explain the recommended way to access attributes. You should have read the previous parts of the series before reaching here, as this is a continuation.

Constant in a Class
To have a constant in a class, use the constant function. For a public attribute, just use a public constant function. For a private attribute, use a code reference constant function. See illustrations below.

Accessors
You have the get() accessor for a class. It is similar to the protected getParent() method for a subclass; however, instead of restricting access to the subclass, the access is restricted to the parent class.

You have the set() accessor for a class. This too is similar to the protected getParent() method for a subclass; however, instead of restricting access to the subclass, the access is restricted to the parent class. See illustrations below.

Code illustrations
The following code illustrates how to code and access a public constant, pi and a public attribute:

use strict;

    {package TheCla;

        sub pi () {3.14159};

        sub get
            {
                return undef unless (ref($_[0]) eq "TheCla");
                return $_[0]->{$_[1]};
            }

        sub new
            {
                bless {word1 =>"pen", word2=>"book"}, $_[0];
            }
    }

    my $obj = TheCla->new();
    print $obj->pi, "\n";
    print $obj->get("word1");

The output is:

3.14159
pen

To obtain the value of the constant, you just call the constant method in the normal way. To obtain the value of a public attribute, you should code and use the get() method. The get() method restricts access only to its class’ object (not to the child object). Note the first statement in the get() method definition, that has, TheCla, which is the name of its class. The return statement of the get() method definition, uses the object reference to access the blessed hash. So, you cannot use the class name with this method to access the blessed hash.

You call the get() method outside the class description, with the object reference. The argument is the name of the attribute (in quotes).

The following code illustrates how to code and access a private constant, pi and a private attribute:

use strict;

    {package TheCla;

        my %ha = (wordA => "pencil", wordB => "ruler");
        $ha{pi} = sub () {3.14159};

        sub pi () {3.14159};

        sub get
            {
                return undef unless (ref($_[0]) eq "TheCla");
                return $ha{$_[1]};
            }

        sub new
            {
                bless {word1 =>"pen", word2=>"book"}, $_[0];
            }
    }

    my $obj = TheCla->new();
    print &{$obj->get("pi")}, "\n";
    print $obj->get("wordA");

The output is:

3.14159
pencil

The get function here restricts access only to the class object. So, the class name cannot be used to access the attributes. This particular get method can be used to access the code reference of any private method, but remember, that the code reference has to be dereferenced as with the private constant here.

The following code illustrates how to access a private and a public attribute:

use strict;

    {package TheCla;

        my %ha = (wordA => "pencil", wordB => "ruler");

        sub pi () {3.14159};

        sub get
            {
                return undef unless (ref($_[0]) eq "TheCla");
                if (exists $ha{$_[1]})
                    {
                        return $ha{$_[1]};
                    }
                elsif (exists $_[0]->{$_[1]})
                    {
                        return $_[0]->{$_[1]};
                    }
            }

        sub new
            {
                bless {word1 =>"pen", word2=>"book"}, $_[0];
            }
    }

    my $obj = TheCla->new();
    print $obj->get("wordA"), "\n";
    print $obj->get("word1");

The output is:

pencil
pen

The get method here restricts access only to the class object. The method definition checks if the attribute (key of hash) exists in the private hash and returns the value if it exists. It alternatively checks if the attribute (key of hash) exists in the blessed hash, and returns the value if it exists.

The following code sets (changes) the value of a private variable and the value of a public variable:

use strict;

    {package TheCla;

        my %ha = (wordA => "pencil", wordB => "ruler");

        sub pi () {3.14159};

        sub set
            {
                return undef unless (ref($_[0]) eq "TheCla");
                if (exists $_[3] eq "private")
                    {
                        $ha{$_[1]} = $_[2];
                        return $ha{$_[1]};
                    }
                else
                    {
                        $_[0]->{$_[1]} = $_[2];
                        return $_[0]->{$_[1]};
                    }
            }

        sub new
            {
                bless {word1 =>"pen", word2=>"book"}, $_[0];
            }
    }

    my $obj = TheCla->new();
    my $priv = $obj->set("wordA", "eraser", "private");
    my $pub = $obj->set("word1", "ink");
    print $priv, "\n", $pub;

The output is:

eraser
ink

The set() method has an optional third argument, which is “private”. If this argument is present, the method sets the private hash. If it is absent, it sets the public blessed hash. The method allows access only to objects of the class. In fact no object from any other class can access the attributes of this class. If the key did not exists in any of the hashes, the element is added. The set() method returns the new value.

Note that the get() and set() methods of the class are public methods. However, they restrict access to the attributes only to objects of the class (not of a different class).

Typical get() Method
A typical get method for private and public attributes should be as follows:

        sub get
         {
                return undef unless (ref($_[0]) eq "TheCla");
                if (exists $ha{$_[1]})
                 {
                        return $ha{$_[1]};
                 }
                elsif (exists $_[0]->{$_[1]})
                 {
                        return $_[0]->{$_[1]};
                 }
         }

Typical set() Method
A typical set method for private and public attributes should be as follows:

        sub set
         {
                return undef unless (ref($_[0]) eq "TheCla");
                if (exists $_[3] eq "private")
                 {
                        $ha{$_[1]} = $_[2];
                        return $ha{$_[1]};
                 }
                else
                 {
                        $_[0]->{$_[1]} = $_[2];
                        return $_[0]->{$_[1]};
                 }
         }

The reason why, access has been restricted to the object of the class and not the class (name), is because in many situations, it is the attributes of the object that are accessed and not the variables of the package. When you want to access the variables of the package (class description) use the static approach – see later.

That is it for this part of the series. We stop here and continue in the next part.

Chrys

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