Intermediate Perl, 2nd Edition

Chapter 18. Object Destruction

In Chapters 13 and 15, we looked at basic object creation and manipulation. In this chapter, we’ll look at an equally important topic: what happens when objects go away. In Perl, we call the process of cleaning up an object destroying it.

As we showed in Chapter 5, when the last reference to a Perl data structure goes away, Perl automatically reclaims the memory of that data structure, including destroying any links to other data. Of course, that in turn may cause Perl to destroy other (“contained”) structures as well.

By default, objects work in this manner because objects use the same reference-count–based garbage collection to make more complex objects. Perl destroys an object built with a hash reference when the last reference to that hash goes away. If hash values are also references, they’re similarly removed, possibly causing further destruction.

Cleaning Up After Ourselves

Suppose our object uses a temporary file to hold data that doesn’t fit entirely in memory. The object can include a filehandle to a temporary file in its instance data. While the normal object destruction sequence will properly close the handle, we still have the temporary file on disk unless we take further action.

To do the proper cleanup operations when Perl destroys an object, we need to know when that happens. Thankfully, Perl provides such notification upon request. We can request this notification by giving the object a DESTROY method.

When the last reference to an object, ...