As a 75 year old C/C++ programmer, I am wedded to strongly typed languages. This makes it difficult for me to learn perl. In particular, the question of the defined/undefined attribute is very difficult for me to grok. Here's the deal:
My introductory perl book tells me that an undefined numeric variable returns 0 and an undefined array variable returns the empty string. Both values are valid for their respective types. (I am told that I am not supposed to express opinions here, but still I wonder what my program will do with these variable values if I mistakenly thought they were defined.) Anyhow, it is apparent that a perl variable must be implemented with a data structure that contains a defined/undefined indicator. Is this the case?
I have spent a good bit of time cruising the web for an answer, but nobody seems to recognize the issue.
Thanks,
John
I didn't read the perl source and I don't know what else I could have tried.
Yes, Perl variables are backed by internal data structures (called SVs) that include a flag to indicate whether a value is defined. So you're right — Perl keeps track of whether a variable is defined or not. When you use an undefined variable, Perl tries to handle it gracefully: in numeric context it becomes 0, and in string context it becomes an empty string "". But this can easily hide bugs if you're not careful. To catch such mistakes, always use use strict; use warnings; and check with defined($var) when needed.