notes_part_8.txt

---------- Notes on chapter 11 of Beginning Perl (Simon Cozens, 2000). ---------

Perl "functions" are called "subroutines" because they don't return values by
default; a return statement must be explicitly declared.
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Turning references into objects using the "bless" operator:
   bless ( reference, package );
For example:
   bless ( $a, "Person" );
   # $a is now a "Person" reference instead of a hash/array/scalar reference
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Storing attributes:

   my $attribute = "green";
   my $object = \$attribute;
   bless ( $object, "Simple" );
   # Now we have class Simple that stores an object with attribute "green"
   
Changing attributes;

   $attribute = $$object;
   $$object = "red";
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Constructors:

Make a hash reference, then bless it into the class.

   package Person;
   sub new {
      my $self = {};
      bless ($self, "Person");
      return $self;
   }

Now to create a Person:

   my $person = Person->new();

Considering inheritence: shift without any arguments means shift @_, so it
takes the first element of the argument array. This gives us the first thing we
passed, the class name.

   sub new {
      my $class = shift;
      my $self = {};
      bless($self, $class);
      return $self;
   }

Providing attributes: we allow the user to specify the attributes of their newly
created object with a hash by initializing $self to be a hash reference to the
argument array @_.

   sub new {
      my $class = shift;
      my $self = {@_};
      bless($self, $class);
      return $self;
   }

Now we can initialize a person with attributes using the constructor:
   my $object = Person->new(
      surname => "Ayat",
      forename => "Naim",
      phone => "3105551234"
   };
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Accessors: use shift to get the object passed into the accessor, then take the
relevant hash entry.

   sub surname {
      my $self = shift;
      return $self->{surname};
   }

Syntax note:
   $reference->{surname}; # Accesses a hash reference
   $reference->[3]; # Accesses an array reference
   $reference->(); # Accesses a function reference
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Generating an error message when an accessor is called with something that isn't
an object:

   use Carp;

   sub surname {
      my $self = shift;
      unless (ref $self) {
         croak "Should call surname() With an object, not a class";
      }
      return $self->{surname};
   }
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Get-Set Methods:

   sub surname {
      my $self = shift;
      unless (ref $self) {
         croak "Should call surname() With an object, not a class";
      }

      # Receive more data
      my $data = shift;
      # Set the address if there's any data there
      $self->{address} = $data if defined $data;

      return $self->{surname};
   }
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Class Attributes: say we want a population count for each new person created
during program execution.

   package Person;
   my $Population = 0;
   sub new {
      my $class = shift;
      my $self = {@_};
      bless($self, $class);
      $Population++;
      return $self;
   }

   sub headcount { $Population; }

   print "Population is: ", Person->headcount;
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Private methods are denoted with an underscore at the beginning of their names.
This is simply convention; it means nothing to the interpreter.

Put the initializer statement in the constructor:

   my @Everyone;

   sub new {
      my $class = shift;
      my $self = {@_};
      bless($self, $class);
      $self->_init;;
      return $self;
   }

Create the _init sub:
   sub _init {
      my $self = shift;
      push @Everyone, $self;
      carp "New object created."
   }
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Inheritance:

To build a new employee class based on person:

   package Employee;
   use Person9.pm;

   our @ISA = "Person";

Adding new methods: nothing special, just add methods.

   sub raise {
      my $self = shift;
      my $newsalary = $self->salary + 2000;
      $self->salary($newsalary);
      return $self;
   }

Overriding methods: provide a new init:

   sub _init {
      my $self = shift;
      my $employer = $self->employer || "unknown";
      unless (ref $employer) {
         my $new_o = Person->new( surname => $employer );
         $self->employer($new_o);
      }
      # This means "call this method in the superclass"
      $self->SUPER::_init();
   }

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Ties: to tie a variable to a class:
   tie $variable, 'Class', @parameters;
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