Programming Perl, 3rd Edition

Method Invocation

If you were to boil down all of object-oriented programming into one quintessential notion, it would be abstraction. It's the single underlying theme you'll find running through all those 10-dollar words that OO enthusiasts like to bandy about, like polymorphism and inheritance and encapsulation. We believe in those fancy words, but we'll address them from the practical viewpoint of what it means to invoke methods. Methods lie at the heart of the object system because they provide the abstraction layer needed to implement all these fancy terms. Instead of directly accessing a piece of data sitting in an object, you invoke an instance method. Instead of directly calling a subroutine in some package, you invoke a class method. By interposing this level of indirection between class use and class implementation, the program designer remains free to tinker with the internal workings of the class, with little risk of breaking programs that use it.

Perl supports two different syntactic forms for invoking methods. One uses a familiar style you've already seen elsewhere in Perl, and the second is a form you may recognize from other programming languages. No matter which form of method invocation is used, the subroutine constituting the method is always passed an extra initial argument. If a class is used to invoke the method, that argument will be the name of the class. If an object is used to invoke the method, that argument will be the reference to the object. Whichever ...