perl 5.003_07: pod/perltrap.pod

[perl5.git] / pod / perlobj.pod

=head1 NAME

perlobj - Perl objects

=head1 DESCRIPTION

First of all, you need to understand what references are in Perl.  See
L<perlref> for that.  

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 static 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 {} }

The C<{}> constructs a reference to an anonymous hash containing no 
key/value pairs.  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, since the referenced object itself knows that
it has been blessed, and its 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 (and you should; see
L<perlmod/"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-E<gt>new()> but also
C<$obj-E<gt>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
only be accessed 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 only
belong to one class at a time.  (Although of course it's free to 
inherit methods from many classes.)

A clarification:  Perl objects are blessed.  References are not.  Objects
know which package they belong to.  References do not.  The bless()
function simply uses the reference in order 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 just 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. 

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 a method isn't found, but an AUTOLOAD routine is found, then
that is called on behalf of the missing method.

If neither a method nor an AUTOLOAD routine is found in @ISA, then one
last try is made for the method (or an AUTOLOAD routine) in a class
called UNIVERSAL.  (Several commonly used methods are automatically
supplied in the UNIVERSAL class; see L<"Default UNIVERSAL methods"> for
more details.)  If that doesn't work, Perl finally gives up and
complains.

Perl classes only do method inheritance.  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.

=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 or package it is being invoked on.  There are just two
types of methods, which we'll call static and virtual, in honor of
the two C++ method types they most closely resemble.

A static 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 static
methods.  Many static methods simply ignore their first argument, since
they already know what package they're in, and don't care what package
they were invoked via.  (These aren't necessarily the same, since
static methods follow the inheritance tree just like ordinary virtual
methods.)  Another typical use for static methods is to look up an
object by name:

    sub find {
	my ($class, $name) = @_;
	$objtable{$name};
    }

A virtual 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 static or virtual methods.
We'll use the two methods defined above, the static method to lookup
an object reference and the virtual 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 -E<gt> 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 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 parens 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.

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 merely
wish 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 pseudoclass,
which says to start looking in your base class's @ISA list without having
to explicitly name it:

    $self->SUPER::display('Height', 'Weight');

Please note that the C<SUPER::> construct is I<only> meaningful 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);

=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 sub-class 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 ( [ VERSION ] )

C<VERSION> returns the VERSION number of the class (package). If
an argument is given then it will check that the current version is not 
less that the given argument. This method is normally called as a static
method. This method is also called when the C<VERSION> form of C<use> is
used.


    use A 1.2 qw(some imported subs);
    
    A->require_version( 1.2 );

=item class ()

C<class> returns the class name of its object.

=item is_instance ()

C<is_instance> returns true if its object is an instance of some
class, false if its object is the class (package) itself. Example

    A->is_instance();       # False
    
    $var = 'A';
    $var->is_instance();    # False
    
    $ref = bless [], 'A';
    $ref->is_instance();    # True

=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.

=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 doesn't do nested destruction for you.  If your constructor
reblessed a reference from one of your base classes, your DESTROY may
need to call DESTROY for any base classes that need it.  But this only
applies to reblessed objects--an object reference that is merely
I<CONTAINED> in the current object will be freed and destroyed
automatically when the current object is freed.

=head2 WARNING

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.  The left side of -E<gt> is not so
limited, because it's an infix operator, not a postfix operator.  

That means that below, A and B are equivalent to each other, and C and D
are equivalent, but AB and CD are different:

    A: method $obref->{"fieldname"} 
    B: (method $obref)->{"fieldname"}
    C: $obref->{"fieldname"}->method() 
    D: method {$obref->{"fieldname"}}

=head2 Summary

That's about all there is to it.  Now you just need 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.  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 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 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
C<-DDEBUGGING> was enabled during perl build time.

A more complete garbage collection strategy will be implemented
at a future date.

=head1 SEE ALSO

You should also check out L<perlbot> for other object tricks, traps, and tips, 
as well as L<perlmod> for some style guides on constructing both modules
and classes.