Programming Perl, 4th Edition

Object Constructors

Subroutines can also return references. That may sound trite, but sometimes you are supposed to use a subroutine to create a reference rather than creating the reference yourself. In particular, special subroutines called constructors create and return references to objects. An object is simply a special kind of reference that happens to know which class it’s associated with, and constructors know how to create that association. They do so by taking an ordinary referent and turning it into an object with the bless operator, so we can speak of an object as a blessed reference. There’s nothing religious going on here; since a class acts as a user-defined type, blessing a referent simply makes it a user-defined type in addition to a built-in one. Constructors are often named new—especially by C++ and Java programmers—but they can be named anything in Perl.

Constructors can be called in any of these ways:

$objref = Doggie::–>new(Tail => "short", Ears => "long");  #1
$objref = new Doggie:: Tail => "short", Ears => "long";    #2
$objref = Doggie–>new(Tail => "short", Ears => "long");    #3
$objref = new Doggie Tail => "short", Ears => "long";      #4

The first and second invocations are the same. They both call a function named new that is supplied by the Doggie module. The third and fourth invocations are the same as the first two, but are slightly more ambiguous: the parser will get confused if you define your own subroutine named Doggie. (Which is why people typically stick with ...