In Perl, there is no pre-defined constructor. More generally, in Perl, an object is just a reference associated with a package (the class). bless is the function that does that. This doesn't even have to be in an constructor.

my $obj = {};  # empty anonymous hash reference
bless $obj, "Some::Class";
[download]

With only one argument, bless associates the reference with the current package, whatever that is.

package Some::Class;

my $obj = {};
bless $obj;    # blesses into "Some::Class"
[download]

There is no default constructor in Perl -- though by convention, it's usually called new. When perl calls a method, the invoking object or package name is passed as the first argument.

$obj->method( 'foo' );        # @_ contains ( $obj, 'foo' )
Some::Class->method( 'foo' ); # @_ contains ( 'Some::Class', 'foo' )
[download]

So the constructor needs to look at the first argument to find the class name. If the first argument is a object (checking ref), the class is the result of ref. Otherwise, the first argument is the class name.

package Some::Class;

sub new {
  my $first_arg = shift; # get the first argument
  my $class = ref($first_arg) || $first_arg;  

  my $self = {};         # create a new, anonymous reference;
  bless $self, $class:   # bless it into the class
  return $self;
}
[download]

The variations are mostly shortcuts of all those separate statements. However, if the single-argument form of bless is called, the constructor always blesses into the current package -- which won't work for subclasses. By using the two-argument form, subclasses can inherit new and the object will be properly blessed into the subclass (which is passed as the first argument). Likewise, some people don't like $obj->new() and so don't bother to check ref -- they just require the constructor to only be called as a class method.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.