Perl Tutorials - Herong's Tutorial Examples - Version 5.32, by Dr. Herong Yang

Perl Tutorials - Herong's Tutorial Examples

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

∟bless() - Converting References to Objects

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".

Table of Contents

 About This Book

 Perl on Linux Systems

 ActivePerl on Windows Systems

 Data Types: Values and Variables

 Expressions, Operations and Simple Statements

 User Defined Subroutines

 Perl Built-in Debugger

 Name Spaces and Perl Module Files

 Symbolic (or Soft) References

 Hard References - Addresses of Memory Objects

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

 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

 Typeglob and Importing Identifiers from Other Packages

 String Built-in Functions and Performance

 File Handles and Data Input/Output

 Open Files in Binary Mode

 Open Directories and Read File Names

 File System Functions and Operations

 Converting Perl Script to Executable Binary

 Using DBM Database Files

 Using MySQL Database Server

 Socket Communication Over the Internet

 XML::Simple Module - XML Parser and Generator

 XML Communication Model

 SOAP::Lite - SOAP Server-Client Communication Module

 Perl Programs as IIS Server CGI Scripts

 CGI (Common Gateway Interface)

 XML-RPC - Remote Procedure Call with XML and HTTP

 RPC::XML - Perl Implementation of XML-RPC

 Integrating Perl with Apache Web Server

 CGI.pm Module for Building Web Pages

 LWP::UserAgent and Web Site Testing

 References

 PDF Printing Version

bless() - Converting References to Objects - Updated in 2018, by Dr. Herong Yang