=head1 DESCRIPTION
-First of all, you need to understand what references are in Perl.
+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>.
+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.
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, because the referenced object itself knows that
-it has been blessed, and the reference to it could have been returned
-directly, like this:
+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 = {};
return $self;
}
-In fact, you often see such a thing in more complicated constructors
+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 = {}
+ my $self = {};
bless $self;
$self->initialize();
return $self;
}
If you care about inheritance (and you should; see
-L<perlmod/"Modules: Creation, Use, and Abuse">),
+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
+ 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()
+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:
my $this = shift;
my $class = ref($this) || $this;
my $self = {};
- bless $self, $class
+ bless $self, $class;
$self->initialize();
return $self;
}
the reference is generally treated as an opaque value that may
be accessed only 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 belong
-to only one class at a time. (Although of course it's free to
-inherit methods from many classes.)
+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()
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
+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
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
+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 a method isn't found, but an AUTOLOAD routine is found, then
-that is called on behalf of the missing method.
+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 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.
+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 only 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.
+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 or package it is being invoked on. There are just two
-types of methods, which we'll call class and instance.
-(Sometimes you'll hear these called static and virtual, in honor of
-the two C++ method types they most closely resemble.)
+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 typically class
-methods. Many class methods simply ignore their first argument, because
-they already know what package they're in, and don't care what package
+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
display {find Critter "Fred"} 'Height', 'Weight';
-For C++ fans, there's also a syntax using -E<gt> notation that does exactly
+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");
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
+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,
Critter->new('Bam' x 2), 1.4, 45
-which is unlikely to do what you want.
+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.
-In this case, you can call your method as an ordinary subroutine
+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');
-Note however, that this does not do any inheritance. If you wish
+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:
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:
+with a simple scalar variable containing the method name or a
+reference to the function.
$method = $fast ? "findfirst" : "findbest";
- $fred->$method(@args);
+ $fred->$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
use UNIVERSAL qw(isa);
if(isa($ref, 'ARRAY')) {
- ...
+ #...
}
=item can(METHOD)
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
+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.
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
+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 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 in the following, A and B are equivalent to each other, and
-C and D are equivalent, but A/B and C/D 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 need just to go off and buy a
=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
+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
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
+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
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.
+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.
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