Perl Tutorials - Herong's Tutorial Examples

∟Hard References - Addresses of Memory Objects

∟${EXPR} - Replacing Identifiers by Expressions

This section provides a tutorial example on how to use hard references by replacing identifiers with any expression that returns hard references, like ${EXPR}.

As mentioned in the previous section, if the hard reference of a variable or function is represented by an expression, the expression can be placed in curly brackets {} to replace the variable or function identifier.

The following tutorial program shows you some interesting examples. In the first group, I used scalar variables that contain hard references in the lookups. In the second group, I used hard references directly in the lookups. In the third group, I used an array to store the hard references. 

#- HardRef2.pl
#- Copyright (c) 1999 by Dr. Herong Yang, http://www.herongyang.com/
#
   $foo = 0;
   @foo = (0);
   %foo = (k,0);
   $refs = \$foo;
   $refa = \@foo;
   $refh = \%foo;
   $reff = \&foo;
   @refl = (\$foo, \@foo, $refh, $reff);
   
   ${$refs} = 10; print "$foo\n";           # the scalar of $foo
   @{$refa} = (20); print "$foo[0]\n";      # entire array of @foo
   ${$refa}[0] = 30; print "$foo[0]\n";     # an element of @foo
   @{$refa}[0] = (40); print "$foo[0]\n";   # a slice of @foo
   %{$refh} = ('k',50); print "$foo{k}\n";  # entire hash of %foo
   ${$refh}{'k'} = 60; print "$foo{k}\n";   # an element of %foo
   @{$refh}{'k'} = (70); print "$foo{k}\n"; # a slice of %foo
   &{$reff}(80);                            # calling &foo

   ${\$foo} = 110; print "$foo\n";           
   @{\@foo} = (120); print "$foo[0]\n";    
   ${\@foo}[0] = 130; print "$foo[0]\n";   
   @{\@foo}[0] = (140); print "$foo[0]\n";   
   %{\%foo} = ('k',150); print "$foo{k}\n";
   ${\%foo}{'k'} = 160; print "$foo{k}\n"; 
   @{\%foo}{'k'} = (170); print "$foo{k}\n"; 
   &{\&foo}(180);                          

   ${$refl[0]} = 210; print "$foo\n";       
   @{$refl[1]} = (220); print "$foo[0]\n";
   ${$refl[1]}[0] = 230; print "$foo[0]\n";    
   @{$refl[1]}[0] = (240); print "$foo[0]\n";
   %{$refl[2]} = ('k',250); print "$foo{k}\n";
   ${$refl[2]}{'k'} = 260; print "$foo{k}\n";
   @{$refl[2]}{'k'} = (270); print "$foo{k}\n";
   &{$refl[3]}(280);                       

sub foo {print "$_[0]\n";}

Here is the output of the tutorial program: 

10
20
30
40
50
60
70
80
110
120
130
140
150
160
170
180
210
220
230
240
250
260
270
280

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

 \* - Creating Hard References

 Using Hard References

 $$name - Replacing Identifiers by Scalar Variables

►${EXPR} - Replacing Identifiers by Expressions

 EXPR->* - The Dereference Operator

 $$$name - Nested Hard References

 \$b-\$a - Using Hard References in Other Operations

 Objects (or References) and Classes (or Packages)

 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