method bless
Documentation for method bless
assembled from the following types:
class Mu
From Mu
(Mu) method bless
method bless(* --> Mu)
Low-level object construction method, usually called from within new
, implicitly from the default constructor, or explicitly if you create your own constructor. bless
creates a new object of the same type as the invocant, using the named arguments to initialize attributes and returns the created object.
It is usually invoked within custom new
method implementations:
my = Point.new(-1, 1);
In this case we are declaring new
as a multi method
so that we can still use the default constructor like this: Point.new( x => 3, y => 8 )
. In this case we are declaring this new
method simply to avoid the extra syntax of using pairs when creating the object. self.bless
returns the object, which is in turn returned by new
.
However, in general, implementing a customized new
method might not be the best way of initializing a class, even more so if the default constructor is disabled, since it can make it harder to correctly initialize the class from a subclass. For instance, in the above example, the new
implementation takes two positional arguments that must be passed from the subclass to the superclass in the exact order. That is not a real problem if it's documented, but take into account bless
will eventually be calling BUILD
in the class that is being instantiated. This might result in some unwanted problems, like having to create a BUILD
submethod to serve it correctly:
is Pointmy = Point-with-ID.new(1,2);say .perl;# OUTPUT: «Point-with-ID.new(ID => "*1-2", x => 1, y => 2)»
In this code, bless
, called within Point.new
, is eventually calling BUILD
with the same parameters. We have to create a convoluted way of using the $.ID
attribute using the metaobject protocol so that we can instantiate it and thus serve that new
constructor, which can be called on Point-with-ID
since it is a subclass.
We might have to use something similar if we want to instantiate superclasses. bless
will help us with that, since it is calling across all the hierarchy:
is Strsay Str-with-ID.new("1.1,2e2").ID; # OUTPUT: «0»my = Str-with-ID.new("3,4");say "$enriched-str, , ";# OUTPUT: «3,4, Str-with-ID, 1»
We are enriching Str
with an auto-incrementing ID. We create a new
since we want to initialize it with a string and, besides, we need to instantiate the superclass. We do so using bless
from within new
. bless
is going to call Str.BUILD
. It will *capture* the value it's looking for, the pair value =
$str> and initialize itself. But we have to initialize also the properties of the subclass, which is why within BUILD
we use the previously explained method to initialize $.ID
with the value that is in the %args
variable. As shown in the output, the objects will be correctly initialized with its ID, and will correctly behave as Str
, converting themselves in just the string in the say
statement, and including the ID
property as required.
For more details see the documentation on object construction.