Object-Oriented Perl constructor syntax and named parameters

Question

I'm a little confused about what is going on in Perl constructors. I found these two examples perldoc perlbot.

package Foo;  #In Perl, the constructor is just a subroutine called new. sub new {   #I don't get what this line does at all, but I always see it. Do I need it?   my $type = shift;    #I'm turning the array of inputs into a hash, called parameters.   my %params = @_;    #I'm making a new hash called $self to store my instance variables?   my $self = {};    #I'm adding two values to the instance variables called "High" and "Low".   #But I'm not sure how $params{'High'} has any meaning, since it was an   #array, and I turned it into a hash.   $self->{'High'} = $params{'High'};   $self->{'Low'} = $params{'Low'};    #Even though I read the page on [bless][2], I still don't get what it does.   bless $self, $type; } 

And another example is:

package Bar;  sub new {   my $type = shift;    #I still don't see how I can just turn an array into a hash and expect things   #to work out for me.   my %params = @_;   my $self = [];    #Exactly where did params{'Left'} and params{'Right'} come from?   $self->[0] = $params{'Left'};   $self->[1] = $params{'Right'};    #and again with the bless.   bless $self, $type; } 

And here is the script that uses these objects:

package main;  $a = Foo->new( 'High' => 42, 'Low' => 11 ); print "High=$a->{'High'}\n"; print "Low=$a->{'Low'}\n";  $b = Bar->new( 'Left' => 78, 'Right' => 40 ); print "Left=$b->[0]\n"; print "Right=$b->[1]\n"; 

I've injected the questions/confusion that I've been having into the code as comments.

Answer 1

To answer the main thrust of your question, since a hash can be initialized as a list of key => value pairs, you can send such a list to a function and then assign @_ to a hash. This is the standard way of doing named parameters in Perl.

For example,

sub foo {      my %stuff = @_;     ... }  foo( beer => 'good', vodka => 'great' ); 

This will result in %stuff in subroutine foo having a hash with two keys, beer and vodka, and the corresponding values.

Now, in OO Perl, there's some additional wrinkles. Whenever you use the arrow (->) operator to call a method, whatever was on the left side of the arrow is stuck onto the beginning of the @_ array.

So if you say Foo->new( 1, 2, 3 );

Then inside your constructor, @_ will look like this: ( 'Foo', 1, 2, 3 ).

So we use shift, which without an argument operates on @_ implicitly, to get that first item out of @_, and assign it to $type. After that, @_ has just our name/value pairs left, and we can assign it directly to a hash for convenience.

We then use that $type value for bless. All bless does is take a reference (in your first example a hash ref) and say "this reference is associated with a particular package." Alakazzam, you have an object.

Remember that $type contains the string 'Foo', which is the name of our package. If you don't specify a second argument to bless, it will use the name of the current package, which will also work in this example but will not work for inherited constructors.