Importing from the ‘Exporter’

One advantage of the Exporter module is that the import method it provides is well developed and handles many different situations for us. Even if we decide to provide our own import subroutine we may want to use Exporter too, just for the richness of the features it provides. For example, it accepts regular expressions as well as literal symbol names, which means that we can define a collection of symbols with similar prefixes and then allow them to be imported together rather than individually. Example:
#Imports a collection of symbols all starting with “prefix_”
use Module qw(/^prefix_/);
#Imports all symbols that do not match a given name or regular expression
use Module qw(!green_planet);
#Import anything not beginning with ‘prefix_’
useModule qw(!/^prefix_/);
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Exporting

Having examined the import mechanism from the side of the importer, we can take a look at how modules handle import requests. From the module’s perspective of course, this is exporting. The Exporter module provides a generic import subroutine for modules to configure to their own particular tastes. It handles almost all the possible issues that a module needs to consider, and for many modules it is all they need – they do not actually need to define their own import subroutine.Using the ‘Exporter’
To use the Exporter, a module needs to do three things; use the Exporter, inherit from it, and define the symbols to export.

Example:
# Module.pm
package Module;
use strict;
use Exporter; # use the Exporter
# Inherit from it
@Module::ISA = qw(Exporter);
#define export symbols
@Module::EXPORT = qw(green_planet);
sub green_planet
{
return "Hello World !\n";
}
1;

# Demo.pl
use warnings;
use strict;
useModule;
print green_planet;
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The ‘import’ Mechanism

Perl’s mechanism for importing symbols is simple, elegant, and shockingly ad hoc, all at the same time. In a nutshell, we call a subroutine called import in the package that we want to import symbols from. The import stage is a secondary stage beyond actually reading and compiling a module file, so it is not handled by the require directive; instead, it is a separate explicit step:

require My::Module; #load in the module
My::Module->import; #call the “import” subroutine

Or
import My::Module;

Or
import My::Module if My::Module->can(‘import’);
# ‘can’ is a universal method

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Importing and Exporting

The term ‘importing’ is used for the process of taking symbols from another package and adding them to our own. From the perspective of the module being imported from, it is ‘exporting’, of course.
Either way, the process consists of taking a symbol visible in the namespace of one package and making it visible, without qualification, in another. For example, even if we can see it we would rather not refer to a variable called:
$My::Package::With::A::Long::Name::scalar

We can get the references into a typeglob,
my *scalar = \$My::Package::With::A::Long::Name::scalar;

Likewise, to create an alias for a subroutine:
my *localsub = \$My::Package::With::A::Long::Name::packagesub;

But, all the time we can’t create the references for the symbols in the package. It will be more critical when the package is updated. So, all the packages should have a well-defined interface and that all code that uses it should use that interface. The package, not the use of the package, should dictate what the interface is.

If our requirements for importing subroutines and variables from our module into other packages are simple, we can for the most part ignore the technicalities of Perl’s import mechanism, and use the Exporter module to define our interface.
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Two-Phased Garbage Collection

For most purposes, Perl uses a fast and simple reference-based garbage collection system. For this reason, there's an extra dereference going on at some level, so if you haven't built your Perl executable using your C compiler's -O flag, performance will suffer. If you have built Perl with cc -O, then this probably won't matter.
A more serious concern is that unreachable memory with a non-zero reference count will not normally get freed. Therefore, this is a bad idea:
{
my $a;
$a = \$a;
}
Even thought $a should go away, it can't. When building recursive data structures, you'll have to break the self-reference yourself explicitly if you don't care to leak. For example, here's a self-referential node such as one might use in a sophisticated tree structure:
sub new_node {
my $self = shift;
my $class = ref($self) || $self;
my $node = {};
$node->{LEFT} = $node->{RIGHT} = $node;
$node->{DATA} = [ @_ ];
return bless $node => $class;
}
If you create nodes like that, they (currently) won't go away unless you break their self reference yourself. (In other words, this is not to be construed as a feature, and you shouldn't depend on it.)
Almost.
When an interpreter thread finally shuts down (usually when your program exits), then a rather costly but complete mark-and-sweep style of garbage collection is performed, and everything allocated by that thread gets destroyed. This is essential to support Perl as an embedded or a multithreadable language. For example, this program demonstrates Perl's two-phased garbage collection:
#!/usr/bin/perl
package Subtle;
sub new {
my $test;
$test = \$test;
warn "CREATING " . \$test;
return bless \$test;
}
sub DESTROY {
my $self = shift;
warn "DESTROYING $self";
}
package main;
warn "starting program";
{
my $a = Subtle->new;
my $b = Subtle->new;
$$a = 0; # break selfref
warn "leaving block";
}
warn "just exited block";
warn "time to die...";
exit;
When run as /tmp/test, the following output is produced:
starting program at /tmp/test line 18.
CREATING SCALAR(0x8e5b8) at /tmp/test line 7.
CREATING SCALAR(0x8e57c) at /tmp/test line 7.
leaving block at /tmp/test line 23.
DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13.
just exited block at /tmp/test line 26.
time to die... at /tmp/test line 27.
DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.
Notice that ``global destruction'' bit there? That's the thread garbage collector reaching the unreachable.
Objects are always destructed, even when regular refs aren't and in fact are destructed in a separate pass before ordinary refs just to try to prevent object destructors from using refs that have been themselves destructed. Plain refs are only garbage-collected if the destruct level is greater than 0. You can test the higher levels of global destruction by setting the PERL_DESTRUCT_LEVEL environment variable, presuming -DDEBUGGING was enabled during perl build time.
A more complete garbage collection strategy will be implemented at a future date.
In the meantime, the best solution is to create a non-recursive container class that holds a pointer to the self-referential data structure. Define a DESTROY method for the containing object's class that manually breaks the circularities in the self-referential structure.
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Destructors

When the last reference to an object goes away, the object is automatically destroyed. (This may even be after you exit, if you've stored references in global variables.) If you want to capture control just before the object is freed, you may define a DESTROY method in your class. It will automatically be called at the appropriate moment, and you can do any extra cleanup you need to do. Perl passes a reference to the object under destruction as the first (and only) argument. Beware that the reference is a read-only value, and cannot be modified by manipulating $_[0] within the destructor. The object itself (i.e. the thingy the reference points to, namely ${$_[0]}, @{$_[0]}, %{$_[0]} etc.) is not similarly constrained.
If you arrange to re-bless the reference before the destructor returns, perl will again call the DESTROY method for the re-blessed object after the current one returns. This can be used for clean delegation of object destruction, or for ensuring that destructors in the base classes of your choosing get called. Explicitly calling DESTROY is also possible, but is usually never needed. Do not confuse the foregoing with how objects CONTAINED in the current one are destroyed. Such objects will be freed and destroyed automatically when the current object is freed, provided no other references to them exist elsewhere.
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Method Invocation

There are two ways to invoke a method, one of which you're already familiar with, and the other of which will look familiar. Perl 4 already had an ``indirect object'' syntax that you use when you say
print STDERR "help!!!\n";
This same syntax can be used to call either class or instance methods. We'll use the two methods defined above, the class method to lookup an object reference and the instance method to print out its attributes.
$fred = find Critter "Fred";
display $fred 'Height', 'Weight';
These could be combined into one statement by using a BLOCK in the indirect object slot:
display {find Critter "Fred"} 'Height', 'Weight';
For C++ fans, there's also a syntax using -> notation that does exactly the same thing. The parentheses are required if there are any arguments.
$fred = Critter->find("Fred");
$fred->display('Height', 'Weight');
or in one statement,
Critter->find("Fred")->display('Height', 'Weight');
There are times when one syntax is more readable, and times when the other syntax is more readable. The indirect object syntax is less cluttered, but it has the same ambiguity as ordinary list operators. Indirect object method calls are usually parsed using the same rule as list operators: ``If it looks like a function, it is a function''. (Presuming for the moment that you think two words in a row can look like a function name. C++ programmers seem to think so with some regularity, especially when the first word is ``new''.) Thus, the parentheses of
new Critter ('Barney', 1.5, 70)
are assumed to surround ALL the arguments of the method call, regardless of what comes after. Saying
new Critter ('Bam' x 2), 1.4, 45
would be equivalent to
Critter->new('Bam' x 2), 1.4, 45
which is unlikely to do what you want. Confusingly, however, this rule applies only when the indirect object is a bareword package name, not when it's a scalar, a BLOCK, or a Package:: qualified package name. In those cases, the arguments are parsed in the same way as an indirect object list operator like print, so
new Critter:: ('Bam' x 2), 1.4, 45
is the same as
Critter::->new(('Bam' x 2), 1.4, 45)
There are times when you wish to specify which class's method to use. In this case, you can call your method as an ordinary subroutine call, being sure to pass the requisite first argument explicitly:
$fred = MyCritter::find("Critter", "Fred");
MyCritter::display($fred, 'Height', 'Weight');
Note however, that this does not do any inheritance. If you wish merely to specify that Perl should START looking for a method in a particular package, use an ordinary method call, but qualify the method name with the package like this:
$fred = Critter->MyCritter::find("Fred");
$fred->MyCritter::display('Height', 'Weight');
If you're trying to control where the method search begins and you're executing in the class itself, then you may use the SUPER pseudo class, which says to start looking in your base class's @ISA list without having to name it explicitly:
$self->SUPER::display('Height', 'Weight');
Please note that the SUPER:: construct is meaningful only within the class.
Sometimes you want to call a method when you don't know the method name ahead of time. You can use the arrow form, replacing the method name with a simple scalar variable containing the method name:
$method = $fast ? "findfirst" : "findbest";
$fred->$method(@args);
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A Method is Simply a Subroutine

Unlike say C++, Perl doesn't provide any special syntax for method definition. (It does provide a little syntax for method invocation though. More on that later.) A method expects its first argument to be the object (reference) or package (string) it is being invoked on. There are just two types of methods, which we'll call class and instance. (Sometimes you'll hear these called static and virtual, in honor of the two C++ method types they most closely resemble.)
A class method expects a class name as the first argument. It provides functionality for the class as a whole, not for any individual object belonging to the class. Constructors are typically class methods. Many class methods simply ignore their first argument, because they already know what package they're in, and don't care what package they were invoked via. (These aren't necessarily the same, because class methods follow the inheritance tree just like ordinary instance methods.) Another typical use for class methods is to look up an object by name:
sub find {
my ($class, $name) = @_;
$objtable{$name};
}
An instance method expects an object reference as its first argument. Typically it shifts the first argument into a ``self'' or ``this'' variable, and then uses that as an ordinary reference.
sub display {
my $self = shift;
my @keys = @_ ? @_ : sort keys %$self;
foreach $key (@keys) {
print "\t$key => $self->{$key}\n";
}
}
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A Class is Simply a Package

Unlike say C++, Perl doesn't provide any special syntax for class definitions. You use a package as a class by putting method definitions into the class. There is a special array within each package called @ISA, which says where else to look for a method if you can't find it in the current package. This is how Perl implements inheritance. Each element of the @ISA array is just the name of another package that happens to be a class package. The classes are searched (depth first) for missing methods in the order that they occur in @ISA. The classes accessible through @ISA are known as base classes of the current class. All classes implicitly inherit from class UNIVERSAL as their last base class. Several commonly used methods are automatically supplied in the UNIVERSAL class If a missing method is found in one of the base classes, it is cached in the current class for efficiency. Changing @ISA or defining new subroutines invalidates the cache and causes Perl to do the lookup again.

If neither the current class, its named base classes, nor the UNIVERSAL class contains the requested method, these three places are searched all over again, this time looking for a method named AUTOLOAD(). If an AUTOLOAD is found, this method is called on behalf of the missing method, setting the package global $AUTOLOAD to be the fully qualified name of the method that was intended to be called.

If none of that works, Perl finally gives up and complains.
Perl classes do method inheritance only. Data inheritance is left up to the class itself. By and large, this is not a problem in Perl, because most classes model the attributes of their object using an anonymous hash, which serves as its own little namespace to be carved up by the various classes that might want to do something with the object. The only problem with this is that you can't sure that you aren't using a piece of the hash that isn't already used. A reasonable workaround is to prepend your fieldname in the hash with the package name.
sub bump {
my $self = shift;
$self->{ __PACKAGE__ . ".count"}++;
}
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An Object is Simply a Reference

Unlike say C++, Perl doesn't provide any special syntax for constructors. A constructor is merely a subroutine that returns a reference to something ``blessed'' into a class, generally the class that the subroutine is defined in. Here is a typical constructor:

package Critter;
sub new { bless {} }

That word new isn't special. You could have written a construct this way, too:

package Critter;
sub spawn { bless {} }

In fact, this might even be preferable, because the C++ programmers won't be tricked into thinking that new works in Perl as it does in C++. It doesn't. We recommend that you name your constructors whatever makes sense in the context of the problem you're solving. For example, constructors in the Tk extension to Perl are named after the widgets they create.

One thing that's different about Perl constructors compared with those in C++ is that in Perl, they have to allocate their own memory. (The other things is that they don't automatically call overridden base-class constructors.) The {} allocates an anonymous hash containing no key/value pairs, and returns it The bless() takes that reference and tells the object it references that it's now a Critter, and returns the reference. This is for convenience, because the referenced object itself knows that it has been blessed, and the reference to it could have been returned directly, like this:

sub new {
my $self = {};
bless $self;
return $self;
}

In fact, you often see such a thing in more complicated constructors that wish to call methods in the class as part of the construction:

sub new {
my $self = {};
bless $self;
$self->initialize();
return $self;
}

If you care about inheritance, then you want to use the two-arg form of bless so that your constructors may be inherited:

sub new {
my $class = shift;
my $self = {};
bless $self, $class;
$self->initialize();
return $self;
}

Or if you expect people to call not just CLASS->new() but also $obj->new(), then use something like this. The initialize() method used will be of whatever $class we blessed the object into:

sub new {
my $this = shift;
my $class = ref($this) || $this;
my $self = {};
bless $self, $class;
$self->initialize();
return $self;
}

Within the class package, the methods will typically deal with the reference as an ordinary reference. Outside the class package, the reference is generally treated as an opaque value that may be accessed only through the class's methods.

A constructor may re-bless a referenced object currently belonging to another class, but then the new class is responsible for all cleanup later. The previous blessing is forgotten, as an object may belong to only one class at a time. (Although of course it's free to inherit methods from many classes.) If you find yourself having to do this, the parent class is probably misbehaving, though.

A clarification: Perl objects are blessed. References are not. Objects know which package they belong to. References do not. The bless() function uses the reference to find the object. Consider the following example:

$a = {};
$b = $a;
bless $a, BLAH;
print "\$b is a ", ref($b), "\n";

This reports $b as being a BLAH, so obviously bless() operated on the object and not on the reference.
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Object Oriented Perl

The next series of posts are on Object Oriented Perl!
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Perl Classes

There is no special class syntax in Perl, but a package may act as a class if it provides subroutines to act as methods. Such a package may also derive some of its methods from another class (package) by listing the other package name(s) in its global @ISA array (which must be a package global, not a lexical).

    package Module;  # assumes Module.pm
   use strict;
   use warnings;
   BEGIN {
       print "\n BEGIN \n";
   }
   INIT{
       print "\n INIT \n";
   }
   CHECK{
       print "\n CHECK \n";
   }
   sub func1()  {}   
   sub func2()  {}   
   sub func3()  {}   
   sub func4()  {}   
   END {
       print "\n END \n";
   }
   print "\n BODY \n";   
   1; #Return value from package

To use the package “Module.pm”,

use Module; 
    Module::fun1(); # To call the methods

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Package Constructors and Destructors

Four special subroutines act as package constructors and destructors. These are the BEGIN, CHECK, INIT, and DESTROY routines. The sub is optional for these routines.

A BEGIN subroutine is executed as soon as possible, that is, the moment it is completely defined, even before the rest of the containing file is parsed. You may have multiple BEGIN blocks within a file--they will execute in order of definition. Because a BEGIN block executes immediately, it can pull in definitions of subroutines and such from other files in time to be visible to the rest of the file. Once a BEGIN has run, it is immediately undefined and any code it used is returned to Perl's memory pool. This means you can't ever explicitly call a BEGIN.

An END subroutine is executed as late as possible, that is, after Perl has finished running the program and just before the interpreter is being exited, even if it is exiting as a result of a die() function. (But not if it's polymorphing into another program via exec, or being blown out of the water by a signal--you have to trap that yourself (if you can).) You may have multiple END blocks within a file--they will execute in reverse order of definition; that is: last in, first out (LIFO). END blocks are not executed when you run perl with the -c switch.

Similar to BEGIN blocks, INIT blocks are run just before the Perl runtime begins execution, in ``first in, first out'' (FIFO) order.

Similar to END blocks, CHECK blocks are run just after the Perl compile phase ends and before the run time begins, in LIFO order. CHECK blocks are again useful in the Perl compiler suite to save the compiled state of the program.
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Perl modules

Perl provides a mechanism for alternative namespaces to protect packages from stomping on each other's variables. In fact, there's really no such thing as a global variable in Perl. The package statement declares the compilation unit as being in the given namespace.

A module is just a set of related function in a library file a Perl package with the same name as the file. It is specifically designed to be reusable by other modules or programs. It may do this by providing a mechanism for exporting some of its symbols into the symbol table of any package using it. Or it may function as a class definition and make its semantics available implicitly through method calls on the class and its objects, without explicitly exporting anything. Or it can do a little of both.

A package statement affects only dynamic variables--including those you've used local() on--but not lexical variables created with my(). Typically it would be the first declaration in a file included by the do, require, or use operators. You can switch into a package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by prefixing the identifier with the package name and a double colon: $Package::Variable. If the package name is null, the main package is assumed. That is, $::sail is equivalent to $main::sail.
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Undefined Values

The undefined value is a curious entity, being neither a scalar, list, hash, nor any other data type, which is essentially the point. Although it isn’t strictly speaking a datatype, it can be helpful to think of it as a special datatype with only one possible value (NULL). The concept of a ‘value that is not a value’ is common to many languages. In Perl, the undef function returns an undefined value, performing the same role as NULL does in C – it also undefines variable arguments passed to it, freeing the memory used to store their values. If we declare a variable without initializing it, it automatically takes on the undefined value too. Perl also provides the defined function that tests for the undefined value and allows us to distinguish it from an empty string or numeric zero.

Example:
$a; # Implicit undefined variable created
$b = undef; # Explicit undefined variable created
$a = 1;
print defined($a); # Prints “1”
print defined($b); # Prints “0”
undef $a;
print defined($a); # Prints “0”

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