I got bit by 'exists' causing autovivication again. It seems that every year I forget about it at least once. I would have thoguht this would have been "fixed" in Perl 5.10 since Perl certainly isn't doing what I mean.

So now I'm curious -- what's the rationale behind it? Is it just something hard to code around in the Perl 5 codebase? Or something else? Would it be possible to have perl throw a warning in a future 5.10 (or 5.12) release?

Obligatory code snippets....

# Broken version:
next LASTLOG unless exists $user_by_uid{$uid}->{$host};

# Data::Dumper output when $uid is not in the hash:
$user_by_uid = {
                 '93688' => {},
                 '58684' => {},
                 '58017' => {},
                };

# Fixed version:
next LASTLOG unless exists $user_by_uid{$uid} && exists $user_by_uid{$
+uid}->{$host};
[download]

Update: I'd like to thank everyone for the replies. I was interested in the history of why it was designed/implemented this way and perrin gave me some things to look at in 659433. I'd like to thank ikegami for his answers in the 659446 thread and special thanks to tye for his responses in Re: Why does exists cause autovivication? (myth, mods) and 659643 respectively. Again, thanks everyone!

Update2: I'd especially like to thank demerphq for his reply in: "Re^7: Why does exists cause autovivication?"!

Happy Holidays!

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.