Here are three very simple definitions that you should find reassuring.
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();
$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 rebless 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.
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 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";
}
}
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');
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.