Advanced Perl Programming

[7] For efficiency, Perl doesn't actually delete it; it just sends it to its own free pool and reuses it when you need a new value. It is logically deleted, nevertheless.

The reference counting technique is sometimes referred to as "poor man's garbage collection," in contrast to much more sophisticated techniques used by environments such as LISP, Java, and Smalltalk (though the early versions of Smalltalk used reference counting). The problem is that reference counts take up space, which adds up if you consider that every piece of data in your application has an extra integer associated with it.

Then there is also the problem of circular references. The simplest case is this: $a = \$a;

This is a classic case of narcissism. $a's reference count indicates that something is pointing to it, so it will never get freed. A more practical case of circular references is that of network graphs (each node keeps track of each of its neighbors) or ring buffers (where the last element points to the first one). Modern garbage collection algorithms implemented in Java and Smalltalk can detect circular references and deallocate the entire circular structure if none of the elements are reachable from other variables.

On the other hand, reference counting is simple to understand and implement and makes it easy to integrate Perl with C or C++ code. Please refer to item 2 in the Section 1.8, "Resources" section at the end of the chapter for a comprehensive treatment of garbage collection techniques.

Note that while symbolic references allow you to access variables in an indirect way, no actual reference variables are created. In other words, the reference count of a symbolically accessed variable is not modified. Hence symbolic references are also called soft references, in contrast to hard references, which actually allocate storage to keep track of the indirection.

This is similar to the concept of soft versus hard links in the Unix filesystem. The i-node of a file has its reference count incremented every time someone creates a hard link to that file, so you can't really delete the file's contents until its reference count goes down to zero. A symbolic link, on the other hand, stores only the name of the file and can point to a nonexistent file; you'll never know until you try to open the file using the symbolic link.

1.6.2 Array/Hash References Versus Element References

Recall that there is a distinction between the array as a whole and each of its constituent scalar values. The array's value maintains its own reference count, and each of its elements has its own. When you take a reference to an array, its own reference count is incremented without its elements getting affected, as shown in Figure 1.6.

Figure 1.6: Taking a reference to an array