Re^5: Keys() required to autovivify?

i.e. I expect $h{x} to hold a hash

That's the clue - you expect that, but it isn't. You look at the box, and you are dereferencing that undef value explicitly as a hash. If you access keys/values of an undef value, your dereference happens implicitly and the hashref is created (autovivification):

use strict;
my $f;
$f->{foo} = 'bar'; # okay
my ($h, %new);
%new = %$h;        # not okay
[download]

Said otherwise, if you dereference something, you state that it is a reference, which is different to autovivication of an undef value into something.

Clearer?

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

Comment on Re^5: Keys() required to autovivify? Download Code

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Re^6: Keys() required to autovivify? by jrw (Monk) on Dec 30, 2007 at 23:39 UTC
To me it looks like I'm explictly dereferencing (or casting) undef in both cases. Don't you interpret %{VAL} as an explicit dereferencing of VAL?	[reply]
Re^7: Keys() required to autovivify? by shmem (Chancellor) on Dec 31, 2007 at 12:43 UTC
Of course `%{VAL}` is an explicit dereferencing of VAL. But assignment to a previously undefined hash reference is not. My previous example can be thought of and written as (and is in fact) `use strict; my $f; ($f = {})->{foo} = 'bar'; my %new; %new = %{undef()}; # not okay` [download] I hope that clarifies things. Read perlref - the chapter about symbolic references. By saying `%{''}` you are creating a symbolic reference to the symbol table entry `{''}`. Typeglobs can be empty strings, but not undef, so you cannot create a symbol reference to an undef value: `$\ = $/; $h = ''; ${''}{foo} = 'bar'; print "\$h->{foo}: ", $h->{foo}; print "\$h: '", $h, "'"; print {''}{HASH}; print {$h}{HASH}; $h = undef; print {$h}{HASH}; __END__ $h->{foo}: bar $h: '' HASH(0x885d7a4) HASH(0x885d7a4) Can't use an undefined value as a symbol reference at - line 11.` [download] The construct `%{$h}` being a symbolic reference: `${''}{foo} = 'bar'; $h = ''; %new = %{$h}; print "$_ => $new{$_}\n" for keys %new; __END__ foo => bar` [download] --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply] [d/l] [select]