bless() - Converting References to Objects

<< Objects (or References) and Classes (or Packages)

<< Perl Tutorials - Herong's Tutorial Examples

This section provides a tutorial example on how to use the bless() function to convert (bless) hard references to objects of the specified class. References of anonymous values or anonymous functions are not allowed to be blessed.

In order to use references (hard references, not soft references) as objects of a class (or package), we need to associate references with a class first. This is done by using the bless() function in one of the following syntaxes:

   $rc = bless($reference, class_name);
   $rc = bless($reference);

where "$reference" is a reference of any data type, which will be associated with the specified "class_name" (package name). "$rc" is the returning copy of the same reference. If "class_name" is omitted, the current package name will be used as the class name. bless() can not be used to references of anonymous scalars.

Perl object - A reference that has been blessed with a package name.

To figure out the associated class name of object, you can use the ref($ref) function, which will return different values:

• (class name) - If $ref has been blessed to a class.
• SCALAR - If $ref is not blessed and is a reference to a scalar.
• ARRAY - If $ref is not blessed and is a reference to an array.
• HASH - If $ref is not blessed and is a reference to a hash.
• CODE - If $ref is not blessed and is a reference to a function.

In order to test the bless() function and the ref() function, I wrote this tutorial program:

#- ObjectBless.pl
#- Copyright (c) 1999 by Dr. Herong Yang, http://www.herongyang.com/
#
   $x = 1;
   $a = \$x;                  # Creating a reference for a scalar
   bless($a, MyClass);        # Converting to an object
   print ref($a), "\n";       # $a is an object of MyClass
   print $a, "\n";            # $a is a reference of scalar variable
   print $$a, "\n";           # using $a as reference
   print ref(\$x), "\n";      # ?

   @y = (Hello);
   $b = \@y;                  # Creating a reference for an array
   bless($b, YourClass);      # Converting to an object
   print ref($b), "\n";       # $b is an object of MyClass
   print $b, "\n";            # $b is a reference of array variable
   print $$b[0], "\n";        # using $b as reference
   print ref(\@y), "\n";      # ?

   $c = \(1+1);               # Reference to an anonymous scalar
#   bless($c, HisClass);      # not allowed on anonymous reference
   $d = [1,2,3];              # Reference to an anonymous array
   bless($d, HisClass);       # Converting to an object
   print ref($d), "\n";       # $d is an object of HisClass
   $e = {Jan,31,Feb,28};      # Reference to an anonymous hash
   bless($e, HisClass);       # Converting to an object
   print ref($e), "\n";       # $e is an object of HisClass

   print ref(\$ss), "\n";     # Returning SCALAR
   print ref(\@aa), "\n";     # Returning ARRAY
   print ref(\%hh), "\n";     # Returning HASH
   print ref(\&ff), "\n";     # Returning CODE
   print ref(\(1+1)), "\n";   # Returning SCALAR

Here is the output of the tutorial program:

MyClass
MyClass=SCALAR(0x36b54)
1
MyClass
YourClass
YourClass=ARRAY(0x1830010)
Hello
YourClass
HisClass
HisClass
SCALAR
ARRAY
HASH
CODE
SCALAR

It is interesting to see that ref(\$x) returns "MyClass", because $a has been blessed to "MyClass".

Sections in This Chapter

Basic Concepts of Classes and Objects

Invoking Package Subroutines as Class Methods

bless() - Converting References to Objects

Invoking Package Subroutines as Object Methods

Class Variables and Object Variables

new() Method - Creating Objects by the Class

CalendarClass.pm - A Perl Class Example