perlobj - Perl objects
First you need to understand what references are in Perl. See 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 perltoot and perltooc.
If you're still with us, then here are three very simple definitions that you should find reassuring.
An object is simply a reference that happens to know which class it belongs to.
A class is simply a package that happens to provide methods to deal with object references.
A method is simply a subroutine that expects an object reference (or a package name, for class methods) as the first argument.
We'll cover these points now in more depth.
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 {} }That word 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 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 {} 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 = {};
bless $self;
return $self;
}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 "Modules: Creation, Use, and Abuse" in perlmodlib), 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 the following. (Note that using this to call new() on an instance does not automatically perform any copying. If you want a shallow or deep copy of an object, you'll have to specifically allow for that.) 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 be accessed only through the class's methods.
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() function uses the reference 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.
Unlike 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 @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.
All classes implicitly inherit from class UNIVERSAL as their last base class. Several commonly used methods are automatically supplied in the UNIVERSAL class; see "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 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 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 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"}++;
}