CODEWITHSUNDEEP
perlreferenceautovivification
Eugene Yarmash
Eugene Yarmash

Reputation: 149823

Why does Perl autovivify in this case?

Why does $a become an arrayref? I'm not pushing anything to it.

perl -MData::Dumper -e 'use strict; 1 for @$a; print Dumper $a'
$VAR1 = [];

Upvotes: 4

Views: 717

Answers (4)

Ray Wadkins
Ray Wadkins

Reputation: 914

$a and $b are special variables in Perl (used in sort) and have a special scope of their own. 

perl -MData::Dumper -e 'use strict; 1 for @$c; print Dumper $c'

produces 

Global symbol "$c" requires explicit package name at -e line 1.
Global symbol "$c" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.

Upvotes: 0

daotoad
daotoad

Reputation: 27183

It is because the for loop treats contents of @$a as lvalues--something that you can assign to. Remember that for aliases the contents of the array to $_. It appears that the act of looking for aliasable contents in @$a, is sufficient to cause autovivification, even when there are no contents to alias.

This effect of aliasing is consistent, too. The following also lead to autovivification:

  • map {stuff} @$a;
  • grep {stuff} @$a;
  • a_subroutine( @$a);

If you want to manage autovivification, you can use the eponymous pragma to effect lexical controls.

Upvotes: 8

brian d foy
brian d foy

Reputation: 132832

When you treat a scalar variable whose value is undef as any sort of reference, Perl makes the value the reference type you tried to use. In this case, $a has the value undef, and when you use @$a, it has to autovivify an array reference in $a so you can dereference it as an array reference.

Upvotes: 3

Alan Haggai Alavi
Alan Haggai Alavi

Reputation: 74252

$a becomes an ARRAY reference due to Perl's autovivification feature.

Upvotes: 0

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