What exactly does Perl's "bless" do?

user_47145 0
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I understand one uses the "bless" keyword in Perl inside a class's "new" method:

sub new {
    my $self = bless { };
    return $self;
}

But what exactly is "bless" doing to that hash reference ?

perl
8 Answers

In general, bless associates an object with a class.

package MyClass;
my $object = { };
bless $object, "MyClass";

Now when you invoke a method on $object, Perl know which package to search for the method.

If the second argument is omitted, as in your example, the current package/class is used.

For the sake of clarity, your example might be written as follows:

sub new { 
  my $class = shift; 
  my $self = { }; 
  bless $self, $class; 
}

EDIT: See kixx's good answer for a little more detail.

Gordon_Wilson_230710

bless associates a reference with a package.

It doesn't matter what the reference is to, it can be to a hash (most common case), to an array (not so common), to a scalar (usually this indicates an inside-out object), to a regular expression, subroutine or TYPEGLOB (see the book Object Oriented Perl: A Comprehensive Guide to Concepts and Programming Techniques by Damian Conway for useful examples) or even a reference to a file or directory handle (least common case).

The effect bless-ing has is that it allows you to apply special syntax to the blessed reference.

For example, if a blessed reference is stored in $obj (associated by bless with package "Class"), then $obj->foo(@args) will call a subroutine foo and pass as first argument the reference $obj followed by the rest of the arguments (@args). The subroutine should be defined in package "Class". If there is no subroutine foo in package "Class", a list of other packages (taken form the array @ISA in the package "Class") will be searched and the first subroutine foo found will be called.

kixx_112600

Short version: it's marking that hash as attached to the current package namespace (so that that package provides its class implementation).

user_475290

What specifically distinguishes a bless-ed referent internally is that the SV for the reference (stored in the scalar) picks up an additional FLAGS value (OBJECT), and also a STASH which carries the package name (with a few other differences)

perl -MDevel::Peek -wE'
    package Pack  { sub func { return { a=>1 } } }; 
    package Class { sub new  { return bless { A=>10 } } }; 
    $vp  = Pack::func(); print Dump $vp;   say"---"; 
    $obj = Class->new;   print Dump $obj'

Prints, with the same (and irrelevant for this) parts suppressed

SV = IV(0x12d5530) at 0x12d5540
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x12a5a68
  SV = PVHV(0x12ab980) at 0x12a5a68
    REFCNT = 1
    FLAGS = (SHAREKEYS)
    ...
      SV = IV(0x12a5ce0) at 0x12a5cf0
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 1
---
SV = IV(0x12cb8b8) at 0x12cb8c8
  REFCNT = 1
  FLAGS = (PADMY,ROK)
  RV = 0x12c26b0
  SV = PVHV(0x12aba00) at 0x12c26b0
    REFCNT = 1
    FLAGS = (OBJECT,SHAREKEYS)         <--
    STASH = 0x12d5300   "Class"        <--
    ...
      SV = IV(0x12c26b8) at 0x12c26c8
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 10

With that it is known to the interpreter that

  • this is an object

  • what package it belongs to

and this informs its use.

For example, when dereferencing on that variable is encountered ($obj->name), a sub with that name is sought in the package (or hierarchy), the object is passed as the first argument, etc.

user_46533790
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