Perl Module Basics

UNIVERSAL

All classes in Perl implicitly inherit from a common base class UNIVERSAL. The 

UNIVERSAL class provides three methods that can be used on all objects isa(),

can(), and VERSION().

The isa() method can be used to determine if an object belongs to a particular 

class or any child of that class. This is preferable to using ref() to check the class of 

an object since it works with inheritance. For example, the following code prints  Ok :

my $logger = BOA::Logger::Enhanced >new("logs/boa.log");

print "Ok" if $logger >isa( BOA::Logger );

but this similar code does not:

my $logger = BOA::Logger::Enhanced >new("logs/boa.log");

print "Ok" if ref($logger) eq  BOA::Logger ;

This is because ref() returns the name of the class that the object belongs to which 

is BOA::Logger::Enhanced. Even though BOA::Logger::Enhanced inherits from 

BOA::Logger, that won't make eq return true when comparing them as strings. The 

moral here is simple: Don't use ref() to check the class of objects, use isa() instead.

To check if an object supports a method call, use can(). You can use can() to 

provide support for older versions of modules while still taking advantage

 of the newer features. For example, imagine that at some point in the future 

BOA::Logger::Enhanced adds a method set_color() that sets the color for the next 

line in the log file. This code checks for the availability of the set_color() method 

and calls it if it is available:

if ($logger >can( set_color )) {

   $logger >set_color( blue );

}

$logger >write("This might be blue, or it might not!");

Another way to query the features provided by a module is to use the VERSION()

method. With no arguments, this method looks at the $VERSION variable defined in 

the class package and returns its value. If you pass an argument to VERSION(), then the 

method will check if the class's $VERSION is greater than or equal to the argument 

and die() if it isn't. This form is used by use when use is passed a version number. 

For example, this statement calls BOA::Logger >VERSION(1.1) after BOA::Logger is 

compiled and exits with an error message if the call returns false:

use BOA::Logger 1.1;

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