Re^3: Keys() required to autovivify?

Using $h{x} is sufficient to autovifiy a key/value pair if there is no key/value pair for the key 'x' in the hash %h.

In the first case you give, the key 'x' is added to the hash (the $h{x} bit does that) with the value undef. The %{...} then tries to dereference the undef and generates the error that you see.

The second case is equivalent to:

my $ref = undef;
keys %{$ref}; # or: keys %$ref;
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which, somewhat surprisingly, is fine and is about the same as asking for the keys of an empty hash. I guess this is an element of DWIM coming into play - if you ask for the keys of a hash reference that you haven't gotten around to assigning any key/value pairs to yet, then a list of not keys (and no errors generated) is a pretty reasonable thing to do.

Perl is environmentally friendly - it saves trees

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Re^4: Keys() required to autovivify? by Anonymous Monk on Dec 30, 2007 at 14:14 UTC
But using $h{x} will not add an undef value into $h{x}. It just returns an undef value since $h{x} doesn't exist. I still don't see what magic keys() is using to get around the syntax error message.	[reply]
Re^5: Keys() required to autovivify? by GrandFather (Saint) on Dec 30, 2007 at 20:31 UTC
A hash can't have a key without a value (that's why I refer to key/value pairs). If you reference $h{x} in some fashion (except as a target for exists) then a x/undef pair is created (autovivified) for %h if it didn't exist already. The undef returned by $h{x} is the value associated with the key 'x'. "Adding" an undef value to %h when $h{x} is used is exactly what autovivification does. Perl is environmentally friendly - it saves trees	[reply]
Re^6: Keys() required to autovivify? by jrw (Monk) on Dec 30, 2007 at 23:59 UTC
Autovivification (in the case of a hash) is when a key is added to a hash where previously that key wasn't there. Just using $h{x}, as in $val = $h{x} does not add a new key to the hash. It simply assigns the default undef value to $val. Referencing through a key does autovivify the all but the last key: `#!/usr/bin/perl my %h; my $val = $h{x}{y}; use Data::Dumper; print Dumper \%h;` [download] Output: `$VAR1 = { 'x' => {} };` [download] Apparently, not only does $h{x} not autovivify $h{x}, but even %{$h{x}} does not autovivify $h{x} (you have to turn off strict and -w first): `#!/usr/bin/perl my %h; %{$h{x}}; use Data::Dumper; print Dumper \%h;` [download] Output: `$VAR1 = {};` [download] But, using keys() causes %{$h[x}} to autovivify $h{x}. `#!/usr/bin/perl -w use strict; my %h; keys %{$h{x}}; use Data::Dumper; print Dumper \%h;` [download] Output: `$VAR1 = { 'x' => {} };` [download]	[reply] [d/l] [select]