AmigActive 2/AACD 2.iso/AACD/Programming/perlman/man/perlobj.txt

Text File | 1999-09-09 | 16.4 KB | 434 lines

NAME
       perlobj - Perl objects

DESCRIPTION
       First of all, you need to understand what references are
       in Perl.  See the perlref manpage for that.

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

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

       2.  A class is simply a package that happens to provide
           methods to deal with object references.

       3.  A method is simply a subroutine that expects an object
           reference (or a package name, for static methods) as
           the first argument.

       We'll cover these points now in more depth.

       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 {} 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 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 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.

       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.  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.

       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";
               }
           }

       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 -> 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 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 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 SUPER:: construct is 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);

       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
       CONTAINED in the current object will be freed and
       destroyed automatically when the current object is freed.

       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 -> 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"}}

       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.

       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.

SEE ALSO
       You should also check out the perlbot manpage for other
       object tricks, traps, and tips, as well as the perlmod
       manpage for some style guides on constructing both modules
       and classes.