Beefy Boxes and Bandwidth Generously Provided by pair Networks
There's more than one way to do things
 
PerlMonks  

Re^2: Basic Class question

by packetstormer (Monk)
on Aug 21, 2013 at 13:11 UTC ( #1050344=note: print w/ replies, xml ) Need Help??


in reply to Re: Basic Class question
in thread Basic Class question

Ken,

Thanks for this, I see I have a bit of reading to do, which is fine (until now I have figured most things out myself but that won't be an option from now on I think!)

One small question: how come the id method isn't expecting the class name as it's first argument? Is it only the first sub that does this? (I will read up anyway, but that question stood out from your code)

Thanks again.


Comment on Re^2: Basic Class question
Re^3: Basic Class question
by Anonymous Monk on Aug 21, 2013 at 13:13 UTC

    One small question: how come the id method isn't expecting the class name as it's first argument?

    Class methods expect class names (string)

    Instance methods expect objects (blessed reference)

Re^3: Basic Class question
by kcott (Abbot) on Aug 22, 2013 at 10:26 UTC
    "One small question: how come the id method isn't expecting the class name as it's first argument? Is it only the first sub that does this? (I will read up anyway, but that question stood out from your code)"

    If you look at the usage of the id() method (which is an instance method), you'll see INSTANCE->METHOD() (e.g. $obj->id()). INSTANCE ($obj in that example) is the first argument to METHOD().

    If it needs to, an instance method can determine the class of the object with ref (as shown with "print "Class Name:   ", ref $obj;" in my original example).

    Similarly, a class method has the form CLASS->METHOD() and the first argument is CLASS.

    The order in which instance and class methods are defined has no bearing on the type of method they are nor the arguments they receive. A class can have any number of methods: MyDir::Class has two (one of each); MyDir::Class::SubClass has no methods; some other class may have lots of instance methods but no class methods; and so on, there's no rules stipulating what the number or mix must be.

    In many cases, the first interaction you have with a class is to create an instance of that class (i.e. an object to work with). Accordingly, the constructor (a class method) is often the first method defined; however, being defined first doesn't bestow any special characteristics.

    [Veering off-topic a bit here but included to clarify a couple of misconceptions I've seen over the years. (1) Constructors are often called new() but they don't have to be. (2) A class can have more than one constructor. As an example of both of these points, take a look at PDF::API2 which has three constructors (new(), open() and openScalar()) all of which return PDF::API2 objects.]

    I've added two more methods to the MyDir::Class example to demonstrate all of the above. get_instance_count() is a class method that's not special at all: it gets a class as its sole argument and uses it to generate the return value. DESTROY() is an instance method that's a destructor: this is special; it's not something you'll normally need; perlobj: Destructors has details. I've also changed the order of the methods so you now have instance/class/instance/class: this was really just to make a point rather than suggesting some preferred ordering. Also note that while the three methods in the anonymous block needed to be grouped within that block, their order within the block is not important.

    $ perl -Mstrict -Mwarnings -le ' package MyDir::Class; # An instance method sub id { my $self = shift; return $self->{id}; } # Anonymous block: code outside this block cannot see %instance_co +unt { my %instance_count; # Constructor: a class method sub new { my ($class, $args_ref) = @_; ++$instance_count{$class}; return bless $args_ref => $class; } # Destructor: an instance method sub DESTROY { my $self = shift; --$instance_count{ref $self}; return; } # Another class method sub get_instance_count { my $class = shift; return $instance_count{$class} || 0; } } package MyDir::Class::SubClass; use base qw{MyDir::Class}; # no methods: all functionality is inherited package main; sub print_all_instance_counts { print "MyDir::Class instances: ", MyDir::Class::->get_instance_count(); print "MyDir::Class::SubClass instances: ", MyDir::Class::SubClass::->get_instance_count(); } # Starting counts print "*** Counts before any objects are created ***"; print_all_instance_counts(); # Create several objects my @objs = map { MyDir::Class::->new({id => $_}) } "A" .. "D"; my @obj_subs = map { MyDir::Class::SubClass::->new({id => $_}) } " +W" .. "Z"; print "*** Counts after initial objects are created ***"; print_all_instance_counts(); # Destroy some objects undef $objs[-1]; pop @obj_subs; splice @obj_subs, 1, 1; shift @obj_subs; print "*** Counts after some objects are destroyed ***"; print_all_instance_counts(); ' *** Counts before any objects are created *** MyDir::Class instances: 0 MyDir::Class::SubClass instances: 0 *** Counts after initial objects are created *** MyDir::Class instances: 4 MyDir::Class::SubClass instances: 4 *** Counts after some objects are destroyed *** MyDir::Class instances: 3 MyDir::Class::SubClass instances: 1

    -- Ken

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://1050344]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others drinking their drinks and smoking their pipes about the Monastery: (4)
As of 2014-12-22 05:10 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    Is guessing a good strategy for surviving in the IT business?





    Results (110 votes), past polls