Beefy Boxes and Bandwidth Generously Provided by pair Networks
Don't ask to ask, just ask
 
PerlMonks  

comment on

( [id://3333]=superdoc: print w/replies, xml ) Need Help??
When a scalar ($foo, $foo[$i], $foo{$k}) is undefined and it is used as a reference (@$foo, $foo->[$i], ${$foo}[$i], %$foo, $foo->{$k}, ${$foo}{$k}, $$foo), two things can happen.

One possibility is that you get a warning. This occurs when you are trying to read from the dereferenced structure. (Technically, when the reference is used as an rvalue).

>perl -wle"my @x = @$ref; print $ref" Use of uninitialized value in array dereference at -e line 1. Use of uninitialized value in print at -e line 1.

The other possibility is that the reference is autovivified. That means the appropriate structure (array, hash or scalar) is constructed and a reference to it is stored in the variable. This occurs when you are trying to write to the dereferenced structure. (Technically, when the reference is used as an lvalue).

>perl -wle"@$ref = my @x; print $ref" ARRAY(0x225228)

Note then creating an alias to the dereferenced structure is an lvalue, and results in autovivification.

>perl -wle"sub {}->(@$ref); print $ref" ARRAY(0x225230) >perl -wle"for (@$ref) {} print $ref" ARRAY(0x225228)

In reply to Re: Explaining Autovivication by ikegami
in thread Explaining Autovivication by perly-gates

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • 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.
Log In?
Username:
Password:

What's my password?
Create A New User
Domain Nodelet?
Chatterbox?
and the web crawler heard nothing...

How do I use this?Last hourOther CB clients
Other Users?
Others having a coffee break in the Monastery: (5)
As of 2024-03-29 06:00 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    No recent polls found