perlobj.pod

ARM上的如果你对底层感兴趣

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 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:

    $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 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> in order 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 foregoing 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 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 ``-E<gt>'' 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 to track such subtle bugs down.

The infix arrow notation using ``C<-E<gt>>'' doesn't suffer from either
of these disturbing ambiguities, so we recommend you use it exclusively.

=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.  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.

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>.
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.