/IronPython_Main/Runtime/Tests/LinqDlrTests/testenv/perl/lib/pod/perlobj.pod

=head1 NAME



perlobj - Perl objects



=head1 DESCRIPTION



First you need to understand what references are in Perl.

See L<perlref> for that.  Second, if you still find the following

reference work too complicated, a tutorial on object-oriented programming

in Perl can be found in L<perltoot> and L<perltootc>.



If you're still with us, then

here are three very simple definitions that you should find reassuring.



=over 4



=item 1.



An object is simply a reference that happens to know which class it

belongs to.



=item 2.



A class is simply a package that happens to provide methods to deal

with object references.



=item 3.



A method is simply a subroutine that expects an object reference (or

a package name, for class methods) as the first argument.



=back



We'll cover these points now in more depth.



=head2 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 C<new> isn't special.  You could have written

a construct this way, too:



    package Critter;

    sub spawn { bless {} }



This might even be preferable, because the C++ programmers won't

be tricked into thinking that C<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 C<{}> 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;

    }



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 (and you should; see

L<perlmodlib/"Modules: Creation, Use, and Abuse">),

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 C<< CLASS->new() >> but also

C<< $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.



Although a constructor can in theory re-bless a referenced object

currently belonging to another class, this is almost certainly going

to get you into trouble.  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.



=head2 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 C<UNIVERSAL> as their

last base class.  Several commonly used methods are automatically

supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for

more details.



If a missing method is found in a base class, 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.



If you want to stop the AUTOLOAD inheritance say simply



	sub AUTOLOAD;



and the call will die using the name of the sub being called.



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"}++;

    } 



=head2 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 two ways of calling methods, which we'll call class

methods and instance methods.  



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 often

class methods, but see L<perltoot> and L<perltootc> for alternatives.

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";

	}

    }



=head2 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 C<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)



For more reasons why the indirect object syntax is ambiguous, see

L<"WARNING"> below.



There are times when you wish to specify which class's method to use.

Here 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');



Unlike method calls, function calls don't consider inheritance.  If you wish

merely to specify that Perl should I<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 I<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 C<SUPER::> construct is meaningful I<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 or a

reference to the function.



    $method = $fast ? "findfirst" : "findbest";

    $fred->$method(@args);  	    # call by name



    if ($coderef = $fred->can($parent . "::findbest")) {

	$self->$coderef(@args);	    # call by coderef

    }



=head2 WARNING



While indirect object syntax may well be appealing to English speakers and

to C++ programmers, be not seduced!  It suffers from two grave problems.



The first problem is that an indirect object is limited to a name,

a scalar variable, or a block, because it would have to do too much

lookahead otherwise, just like any other postfix dereference in the

language.  (These are the same quirky rules as are used for the filehandle

slot in functions like C<print> and C<printf>.)  This can lead to horribly

confusing precedence problems, as in these next two lines:



    move $obj->{FIELD};                 # probably wrong!

    move $ary[$i];                      # probably wrong!



Those actually parse as the very surprising:



    $obj->move->{FIELD};                # Well, lookee here

    $ary->move([$i]);                   # Didn't expect this one, eh?



Rather than what you might have expected:



    $obj->{FIELD}->move();              # You should be so lucky.

    $ary[$i]->move;                     # Yeah, sure.



The left side of ``->'' is not so limited, because it's an infix operator,

not a postfix operator.  



As if that weren't bad enough, think about this: Perl must guess I<at

compile time> whether C<name> and C<move> above are functions or methods.

Usually Perl gets it right, but when it doesn't it, you get a function

call compiled as a method, or vice versa.  This can introduce subtle

bugs that are hard to unravel.  For example, calling a method C<new>

in indirect notation--as C++ programmers are so wont to do--can

be miscompiled into a subroutine call if there's already a C<new>

function in scope.  You'd end up calling the current package's C<new>

as a subroutine, rather than the desired class's method.  The compiler

tries to cheat by remembering bareword C<require>s, but the grief if it

messes up just isn't worth the years of debugging it would likely take

you to track such subtle bugs down.



The infix arrow notation using ``C<< -> >>'' doesn't suffer from either

of these disturbing ambiguities, so we recommend you use it exclusively.



=head2 Default UNIVERSAL methods



The C<UNIVERSAL> package automatically contains the following methods that

are inherited by all other classes:



=over 4



=item isa(CLASS)



C<isa> returns I<true> if its object is blessed into a subclass of C<CLASS>



C<isa> is also exportable and can be called as a sub with two arguments. This

allows the ability to check what a reference points to. Example



    use UNIVERSAL qw(isa);



    if(isa($ref, 'ARRAY')) {

    	#...

    }



=item can(METHOD)



C<can> checks to see if its object has a method called C<METHOD>,

if it does then a reference to the sub is returned, if it does not then

I<undef> is returned.



=item VERSION( [NEED] )



C<VERSION> returns the version number of the class (package).  If the

NEED argument is given then it will check that the current version (as

defined by the $VERSION variable in the given package) not less than

NEED; it will die if this is not the case.  This method is normally

called as a class method.  This method is called automatically by the

C<VERSION> form of C<use>.



    use A 1.2 qw(some imported subs);

    # implies:

    A->VERSION(1.2);



=back



B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and

C<isa> uses a very similar method and cache-ing strategy. This may cause

strange effects if the Perl code dynamically changes @ISA in any package.



You may add other methods to the UNIVERSAL class via Perl or XS code.

You do not need to C<use UNIVERSAL> to make these methods

available to your program.  This is necessary only if you wish to

have C<isa> available as a plain subroutine in the current package.



=head2 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 C<$_[0]> within the destructor.  The object itself (i.e.

the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>, 

C<%{$_[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 previous discussion with how objects I<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.



=head2 Summary



That's about all there is to it.  Now you need just to go off and buy a

book about object-oriented design methodology, and bang your forehead

with it for the next six months or so.



=head2 Two-Phased Garbage Collection



For most purposes, Perl uses a fast and simple, reference-based

garbage collection system.  That means there's an extra

dereference going on at some level, so if you haven't built

your Perl executable using your C compiler's C<-O> flag, performance

will suffer.  If you I<have> built Perl with C<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 I<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 F</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.  Objects

are destructed in a separate pass before ordinary refs just 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

C<-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.



=head1 SEE ALSO



A kinder, gentler tutorial on object-oriented programming in Perl can

be found in L<perltoot>, L<perlbootc> and L<perltootc>.  You should

also check out L<perlbot> for other object tricks, traps, and tips, as

well as L<perlmodlib> for some style guides on constructing both

modules and classes.