[Appendix B] B.5 Objects

B.5 Objects

Salient points:

A class is a package. There's no keyword such as struct or class to define layout of object.
You choose object representation - object layout is not dictated by you.
No special syntax for constructor. You choose the name of the subroutine that is going to allocate the object and return a blessed or typed reference to that object.

Creating an OO package - Method 1 (see also #19).

The C++ class:

class Employee {
    String _name; int _age; double _salary;
    create (String n, int age) : _name(n), _age(age), _salary(0) {}
    ~Employee {printf ("Ahh ... %s is dying\n", _name)}
    set_salary (double new_salary) { this->_salary = new_salary}
};

becomes:

package Employee;
sub create {                # Allocator and Initializer 
     my ($pkg, $name, $age) = @_;
     # Allocate anon hash, bless it, return it.
     return (bless {name => $name, age=> $age, salary=>0}, $pkg);
}
sub DESTROY {               # destructor (like Java's finalize)
    my $obj = shift;
    print "Ahh ... ", $obj->{name}, " is dying\n";
}
sub set_salary {
    my ($obj, $new_salary) = @_;
    $obj->{salary} = $new_salary; # Remember: $obj is ref-to-hash
    return $new_salary;
}

Using object package:

use Employee;
$emp = Employee->new("Ada", 35);
$emp->set_salary(1000);

Creating OO package - Method 2 (see also #17). Inherit from ObjectTemplate, use the attributes method to declare attribute names, and obtain the constructor new and attribute accessor functions for free:

package Employee;
use ObjectTemplate;
@ISA = ("ObjectTemplate");
attributes("name", "age", "salary");
sub DESTROY {
   my $obj = shift;
   print "Ahh ... ", $obj->name(), " is dying\n";
}
sub set_salary {
    my ($obj, $new_salary) = @_;
    $obj->salary($new_salary); 
}

Class methods:

Employee->print();    # 1. "Arrow notation" used for class method
new Employee ();      # 2. Class method using "Indirect notation".

These two class methods must expect the package name as the first parameter, followed by the rest of the arguments.

Instance methods. There are two ways of invoking methods on an object:
```
$emp->promote();
promote $obj;
```