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.
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.