perlobj.1

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.Sh "Method Invocation"
.IX Xref "invocation method arrow ->"
.IX Subsection "Method Invocation"
For various historical and other reasons, Perl offers two equivalent
ways to write a method call.  The simpler and more common way is to use
the arrow notation:
.PP
.Vb 2
\&    my $fred = Critter\->find("Fred");
\&    $fred\->display("Height", "Weight");
.Ve
.PP
You should already be familiar with the use of the \f(CW\*(C`\->\*(C'\fR operator with
references.  In fact, since \f(CW$fred\fR above is a reference to an object,
you could think of the method call as just another form of
dereferencing.
.PP
Whatever is on the left side of the arrow, whether a reference or a
class name, is passed to the method subroutine as its first argument.
So the above code is mostly equivalent to:
.PP
.Vb 2
\&    my $fred = Critter::find("Critter", "Fred");
\&    Critter::display($fred, "Height", "Weight");
.Ve
.PP
How does Perl know which package the subroutine is in?  By looking at
the left side of the arrow, which must be either a package name or a
reference to an object, i.e. something that has been blessed to a
package.  Either way, that's the package where Perl starts looking.  If
that package has no subroutine with that name, Perl starts looking for
it in any base classes of that package, and so on.
.PP
If you need to, you \fIcan\fR force Perl to start looking in some other package:
.PP
.Vb 2
\&    my $barney = MyCritter\->Critter::find("Barney");
\&    $barney\->Critter::display("Height", "Weight");
.Ve
.PP
Here \f(CW\*(C`MyCritter\*(C'\fR is presumably a subclass of \f(CW\*(C`Critter\*(C'\fR that defines
its own versions of \fIfind()\fR and \fIdisplay()\fR.  We haven't specified what
those methods do, but that doesn't matter above since we've forced Perl
to start looking for the subroutines in \f(CW\*(C`Critter\*(C'\fR.
.PP
As a special case of the above, you may use the \f(CW\*(C`SUPER\*(C'\fR pseudo-class to
tell Perl to start looking for the method in the packages named in the
current class's \f(CW@ISA\fR list.
.IX Xref "SUPER"
.PP
.Vb 2
\&    package MyCritter;
\&    use base \*(AqCritter\*(Aq;    # sets @MyCritter::ISA = (\*(AqCritter\*(Aq);
\&
\&    sub display { 
\&        my ($self, @args) = @_;
\&        $self\->SUPER::display("Name", @args);
\&    }
.Ve
.PP
It is important to note that \f(CW\*(C`SUPER\*(C'\fR refers to the superclass(es) of the
\&\fIcurrent package\fR and not to the superclass(es) of the object. Also, the
\&\f(CW\*(C`SUPER\*(C'\fR pseudo-class can only currently be used as a modifier to a method
name, but not in any of the other ways that class names are normally used,
eg:
.IX Xref "SUPER"
.PP
.Vb 3
\&    something\->SUPER::method(...);      # OK
\&    SUPER::method(...);                 # WRONG
\&    SUPER\->method(...);                 # WRONG
.Ve
.PP
Instead of a class name or an object reference, you can also use any
expression that returns either of those on the left side of the arrow.
So the following statement is valid:
.PP
.Vb 1
\&    Critter\->find("Fred")\->display("Height", "Weight");
.Ve
.PP
and so is the following:
.PP
.Vb 1
\&    my $fred = (reverse "rettirC")\->find(reverse "derF");
.Ve
.PP
The right side of the arrow typically is the method name, but a simple 
scalar variable containing either the method name or a subroutine 
reference can also be used.
.Sh "Indirect Object Syntax"
.IX Xref "indirect object syntax invocation, indirect indirect"
.IX Subsection "Indirect Object Syntax"
The other way to invoke a method is by using the so-called \*(L"indirect
object\*(R" notation.  This syntax was available in Perl 4 long before
objects were introduced, and is still used with filehandles like this:
.PP
.Vb 1
\&   print STDERR "help!!!\en";
.Ve
.PP
The same syntax can be used to call either object or class methods.
.PP
.Vb 2
\&   my $fred = find Critter "Fred";
\&   display $fred "Height", "Weight";
.Ve
.PP
Notice that there is no comma between the object or class name and the
parameters.  This is how Perl can tell you want an indirect method call
instead of an ordinary subroutine call.
.PP
But what if there are no arguments?  In that case, Perl must guess what
you want.  Even worse, it must make that guess \fIat compile time\fR.
Usually Perl gets it right, but when it doesn't you get a function
call compiled as a method, or vice versa.  This can introduce subtle bugs
that are hard to detect.
.PP
For example, a call to a method \f(CW\*(C`new\*(C'\fR in indirect notation \*(-- as \*(C+
programmers are wont to make \*(-- can be miscompiled into a subroutine
call if there's already a \f(CW\*(C`new\*(C'\fR function in scope.  You'd end up
calling the current package's \f(CW\*(C`new\*(C'\fR as a subroutine, rather than the
desired class's method.  The compiler tries to cheat by remembering
bareword \f(CW\*(C`require\*(C'\fRs, but the grief when it messes up just isn't worth the
years of debugging it will take you to track down such subtle bugs.
.PP
There is another problem with this syntax: the 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 \f(CW\*(C`print\*(C'\fR and \f(CW\*(C`printf\*(C'\fR.)
This can lead to horribly confusing precedence problems, as in these
next two lines:
.PP
.Vb 2
\&    move $obj\->{FIELD};                 # probably wrong!
\&    move $ary[$i];                      # probably wrong!
.Ve
.PP
Those actually parse as the very surprising:
.PP
.Vb 2
\&    $obj\->move\->{FIELD};                # Well, lookee here
\&    $ary\->move([$i]);                   # Didn\*(Aqt expect this one, eh?
.Ve
.PP
Rather than what you might have expected:
.PP
.Vb 2
\&    $obj\->{FIELD}\->move();              # You should be so lucky.
\&    $ary[$i]\->move;                     # Yeah, sure.
.Ve
.PP
To get the correct behavior with indirect object syntax, you would have
to use a block around the indirect object:
.PP
.Vb 2
\&    move {$obj\->{FIELD}};
\&    move {$ary[$i]};
.Ve
.PP
Even then, you still have the same potential problem if there happens to
be a function named \f(CW\*(C`move\*(C'\fR in the current package.  \fBThe \f(CB\*(C`\->\*(C'\fB
notation suffers from neither of these disturbing ambiguities, so we
recommend you use it exclusively.\fR  However, you may still end up having
to read code using the indirect object notation, so it's important to be
familiar with it.
.Sh "Default \s-1UNIVERSAL\s0 methods"
.IX Xref "UNIVERSAL"
.IX Subsection "Default UNIVERSAL methods"
The \f(CW\*(C`UNIVERSAL\*(C'\fR package automatically contains the following methods that
are inherited by all other classes:
.IP "isa(\s-1CLASS\s0)" 4
.IX Xref "isa"
.IX Item "isa(CLASS)"
\&\f(CW\*(C`isa\*(C'\fR returns \fItrue\fR if its object is blessed into a subclass of \f(CW\*(C`CLASS\*(C'\fR
.Sp
You can also call \f(CW\*(C`UNIVERSAL::isa\*(C'\fR as a subroutine with two arguments.  Of
course, this will do the wrong thing if someone has overridden \f(CW\*(C`isa\*(C'\fR in a
class, so don't do it.
.Sp
If you need to determine whether you've received a valid invocant, use the
\&\f(CW\*(C`blessed\*(C'\fR function from Scalar::Util:
.IX Xref "invocant blessed"
.Sp
.Vb 3
\&    if (blessed($ref) && $ref\->isa( \*(AqSome::Class\*(Aq)) {
\&        # ...
\&    }
.Ve
.Sp
\&\f(CW\*(C`blessed\*(C'\fR returns the name of the package the argument has been
blessed into, or \f(CW\*(C`undef\*(C'\fR.
.IP "can(\s-1METHOD\s0)" 4
.IX Xref "can"
.IX Item "can(METHOD)"
\&\f(CW\*(C`can\*(C'\fR checks to see if its object has a method called \f(CW\*(C`METHOD\*(C'\fR,
if it does then a reference to the sub is returned, if it does not then
\&\fIundef\fR is returned.
.Sp
\&\f(CW\*(C`UNIVERSAL::can\*(C'\fR can also be called as a subroutine with two arguments.  It'll
always return \fIundef\fR if its first argument isn't an object or a class name.
The same caveats for calling \f(CW\*(C`UNIVERSAL::isa\*(C'\fR directly apply here, too.
.IP "\s-1VERSION\s0( [\s-1NEED\s0] )" 4
.IX Xref "VERSION"
.IX Item "VERSION( [NEED] )"
\&\f(CW\*(C`VERSION\*(C'\fR returns the version number of the class (package).  If the
\&\s-1NEED\s0 argument is given then it will check that the current version (as
defined by the \f(CW$VERSION\fR variable in the given package) not less than
\&\s-1NEED\s0; 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
\&\f(CW\*(C`VERSION\*(C'\fR form of \f(CW\*(C`use\*(C'\fR.
.Sp
.Vb 3
\&    use A 1.2 qw(some imported subs);
\&    # implies:
\&    A\->VERSION(1.2);
.Ve
.PP
\&\fB\s-1NOTE:\s0\fR \f(CW\*(C`can\*(C'\fR directly uses Perl's internal code for method lookup, and
\&\f(CW\*(C`isa\*(C'\fR uses a very similar method and cache-ing strategy. This may cause
strange effects if the Perl code dynamically changes \f(CW@ISA\fR in any package.
.PP
You may add other methods to the \s-1UNIVERSAL\s0 class via Perl or \s-1XS\s0 code.
You do not need to \f(CW\*(C`use UNIVERSAL\*(C'\fR to make these methods
available to your program (and you should not do so).
.Sh "Destructors"
.IX Xref "destructor DESTROY"
.IX Subsection "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 \s-1DESTROY\s0 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 \f(CW$_[0]\fR within the destructor.  The object itself (i.e.
the thingy the reference points to, namely \f(CW\*(C`${$_[0]}\*(C'\fR, \f(CW\*(C`@{$_[0]}\*(C'\fR, 
\&\f(CW\*(C`%{$_[0]}\*(C'\fR etc.) is not similarly constrained.
.PP
Since \s-1DESTROY\s0 methods can be called at unpredictable times, it is
important that you localise any global variables that the method may
update.  In particular, localise \f(CW$@\fR if you use \f(CW\*(C`eval {}\*(C'\fR and
localise \f(CW$?\fR if you use \f(CW\*(C`system\*(C'\fR or backticks.
.PP
If you arrange to re-bless the reference before the destructor returns,
perl will again call the \s-1DESTROY\s0 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 \s-1DESTROY\s0 is also possible,
but is usually never needed.
.PP
Do not confuse the previous discussion with how objects \fI\s-1CONTAINED\s0\fR 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.
.Sh "Summary"
.IX Subsection "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.
.Sh "Two-Phased Garbage Collection"
.IX Xref "garbage collection GC circular reference reference, circular DESTROY destructor"
.IX Subsection "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 \f(CW\*(C`\-O\*(C'\fR flag, performance
will suffer.  If you \fIhave\fR built Perl with \f(CW\*(C`cc \-O\*(C'\fR, then this
probably won't matter.
.PP
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:
.PP
.Vb 4
\&    {
\&        my $a;
\&        $a = \e$a;
\&    }
.Ve
.PP
Even thought \f(CW$a\fR \fIshould\fR 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:
.PP
.Vb 7
\&    sub new_node {
\&        my $class = shift;
\&        my $node  = {};
\&        $node\->{LEFT} = $node\->{RIGHT} = $node;
\&        $node\->{DATA} = [ @_ ];
\&        return bless $node => $class;
\&    }
.Ve
.PP
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.)
.PP
Almost.
.PP
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:
.PP
.Vb 2
\&    #!/usr/bin/perl
\&    package Subtle;
\&
\&    sub new {
\&        my $test;
\&        $test = \e$test;
\&        warn "CREATING " . \e$test;
\&        return bless \e$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;
.Ve
.PP
When run as \fI/foo/test\fR, the following output is produced:
.PP
.Vb 8
\&    starting program at /foo/test line 18.
\&    CREATING SCALAR(0x8e5b8) at /foo/test line 7.
\&    CREATING SCALAR(0x8e57c) at /foo/test line 7.
\&    leaving block at /foo/test line 23.
\&    DESTROYING Subtle=SCALAR(0x8e5b8) at /foo/test line 13.
\&    just exited block at /foo/test line 26.
\&    time to die... at /foo/test line 27.
\&    DESTROYING Subtle=SCALAR(0x8e57c) during global destruction.
.Ve
.PP
Notice that \*(L"global destruction\*(R" bit there?  That's the thread
garbage collector reaching the unreachable.
.PP
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 \s-1PERL_DESTRUCT_LEVEL\s0 environment variable, presuming
\&\f(CW\*(C`\-DDEBUGGING\*(C'\fR was enabled during perl build time.
See \*(L"\s-1PERL_DESTRUCT_LEVEL\s0\*(R" in perlhack for more information.
.PP
A more complete garbage collection strategy will be implemented
at a future date.
.PP
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 \s-1DESTROY\s0 method for the containing object's class that manually
breaks the circularities in the self-referential structure.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
A kinder, gentler tutorial on object-oriented programming in Perl can
be found in perltoot, perlboot and perltooc.  You should
also check out perlbot for other object tricks, traps, and tips, as
well as perlmodlib for some style guides on constructing both
modules and classes.