Just another Perl shrine | |
PerlMonks |
perlman:perlobjby gods (Initiate) |
on Aug 25, 1999 at 06:34 UTC ( [id://394]=perlman: print w/replies, xml ) | Need Help?? |
perlobjCurrent Perl documentation can be found at perldoc.perl.org. Here is our local, out-dated (pre-5.6) version:
NAMEperlobj - Perl objects
DESCRIPTIONFirst of all, you need to understand what references are in Perl. See the perlref manpage 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 the perltoot manpage. If you're still with us, then here are three very simple definitions that you should find reassuring.
We'll cover these points now in more depth.
An Object is Simply a ReferenceUnlike 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 {} }
That word
package Critter; sub spawn { bless {} }
In fact, this might even be preferable, because the
C++ programmers won't be tricked into thinking that
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
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 perlman:perlmod), 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
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 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.) 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
$a = {}; $b = $a; bless $a, BLAH; print "\$b is a ", ref($b), "\n";
This reports
A Class is Simply a PackageUnlike say C++, Perl doesn't provide any special syntax for class 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 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
All classes implicitly inherit from class
If a missing method is found in one of the base classes, it is cached in
the current class for efficiency. Changing
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
If none of that works, Perl finally gives up and complains. 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; $self->{ __PACKAGE__ . ".count"}++; }
A Method is Simply a SubroutineUnlike say C++, Perl doesn't provide any special syntax for method definition. (It does provide a little syntax for method invocation though. More on that later.) A method expects its first argument to be the object (reference) or package (string) it is being invoked on. There are just two types of methods, which we'll call class and instance. (Sometimes you'll hear these called static and virtual, in honor of the two C++ method types they most closely resemble.) A class method expects a class name as the first argument. It provides functionality for the class as a whole, not for any individual object belonging to the class. Constructors are typically class methods. Many class methods simply ignore their first argument, because they already know what package they're in, and don't care what package they were invoked via. (These aren't necessarily the same, because class methods follow the inheritance tree just like ordinary instance methods.) Another typical use for class methods is to look up an object by name:
sub find { my ($class, $name) = @_; $objtable{$name}; } An instance 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 InvocationThere 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 class or instance methods. We'll use the two methods defined above, the class method to lookup an object reference and the instance 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 parentheses 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 wish merely 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 pseudo class, which says to start looking in
your base class's
$self->SUPER::display('Height', 'Weight');
Please note that the 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);
Default UNIVERSAL methods
The
NOTE:
You may add other methods to the
UNIVERSAL class via Perl or
XS code. You do not need to
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
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 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.
WARNINGWhile 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 print and 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 at
compile time whether
The infix arrow notation using ``
SummaryThat'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.
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
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
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
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.
SEE ALSOA kinder, gentler tutorial on object-oriented programming in Perl can be found in the perltoot manpage. You should also check out the perlbot manpage for other object tricks, traps, and tips, as well as the perlmodlib manpage for some style guides on constructing both modules and classes. Return to the Library |
|