Hi Geoff,
i like your explanation of the fundamentals of references,
but near the end, you run the risk of spreading confusion:
Geoff wrote on Thu, Jun 26, 2008 at 10:12:00AM -0400:
> ANOTHER way is to say: %myhash = %$data, and then %myhash
> becomes the equivalent of the original %hash.
Better say: a shallow copy, not "the equivalent".
Why is this important?
1. A reference is not a copy.
After you change the original,
the reference points to the new value.
$ perl -e '
> use warnings;
> use strict;
> my %orig = ( key => "oldvalue" );
> my $ref = \%orig;
> print "at first, ref points to: $ref->{key}\n";
> $orig{key} = "newvalue";
> print "after changing orig, ref also points to: $ref->{key}\n";'
at first, ref points to: oldvalue
after changing orig, ref also points to: newvalue
2. Hash assignment does copy.
When you change the original,
the copy does not change.
$ perl -e '
> use warnings;
> use strict;
> my %orig = ( key => "oldvalue" );
> my %copy = %orig;
> $orig{key} = "newvalue";
> print "after changing orig, copy still contains: $copy{key}\n";'
after changing orig, copy still contains: oldvalue
3. The copy is shallow.
So, if references are contained,
they are copied as refernces:
$ perl -e '
> use warnings;
> use strict;
> my $scalar = "oldvalue";
> my $ref = \$scalar;
> my %orig = ( key => $ref );
> print "${$orig{key}}\n"; # caution: not $orig->{key}
> my %copy = %orig;
> $scalar = "newvalue";
> print "${$copy{key}}\n";'
oldvalue
newvalue
You need to be very careful what is refernced and what is copied;
in fact, that's the whole point of references...
Vague terms like "equivalent" are not helpful,
and even "assign" may sometimes be confusing.
[...l
> %anotherhash = %$hashref;
[...]
> When you assign %anotherhash to %$hashref,
Well, no. What you are doing here is assigning the contents
of %$hashref to %anotherhash. Or better, copying the contents
of %$hashref into %anotherhash.
Yours,
Ingo
--
Ingo Schwarze <[EMAIL PROTECTED]> | Software Engineer | Framework Team
Astaro AG | www.astaro.com | 76227 Karlsruhe | Germany
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