passing object references into subs

Avox has asked for the wisdom of the Perl Monks concerning the following question:

I've been trolling and searching, but have not found anything that helps me. If you know of a thread please link me. I have a structure containing a hash. I want to pass the structure into a sub by reference. While in the sub, I want access to the hash. Please cure my stupidity. heres a stripped down example:

use Class::Struct;


struct MyStruct => {
    hash1 => '%',
};

#fill the struct's hash with some data
my $test = MyStruct->new(
                         hash1=>{one=>onesdata}
                       );

use_hash(\$test);

while(($key,$val) = each %{$test->hash1}){
        print "$key=$val\n";
}

sub use_hash{
    my $ref = shift;  #obtain the passed in reference
    $$ref->hash1{two} = 'twosdata';  #PROBLEM
}
[download]

I get an error on the line marked problem. Can anyone explain my error? Many thanks in advance.

Comment on passing object references into subs Download Code

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Re: passing object references into subs by dbp (Pilgrim) on Feb 03, 2003 at 18:50 UTC
Ok. Two problems. First of all, $test is already a reference, so you don't need to pass a reference to the reference to use_hash. If, for some reason, this is what you intended, you're going to need to dereference $ref twice in use_hash. Otherwise change `use_hash(\$test);` [download] to `use_hash($test);` [download] Secondly, assuming you've made the change above, change line 19 to: `%{$ref->hash1}->{'two'} = 'twosdata';` [download] If you haven't made the change above, you'll have to nest a dereference of $ref into this statement, like so: `%{$$ref->hash1}->{'two'} = 'twosdata';` [download]	[reply] [d/l] [select]
Re: Re: passing object references into subs by ihb (Deacon) on Feb 04, 2003 at 17:30 UTC
`%{$ref->hash1}->{'two'} = 'twosdata';` This is not right. In perl561delta it's called a known bug/problem, in perl580delta it's called deprecated behaviour. See http://www.perldoc.com/perl5.6.1/pod/perldelta.html#Arrow-operator-and-arrays and http://www.perldoc.com/perl5.8.0/pod/perldelta.html#Deprecations. This is because it's not "supposed" to work. Just as you don't do `%foo->{key}`. The correct way is to just skip the `%{}`, or use `${}` and emit the second arrow. `$ref->hash1->{'two'} = 'twosdata';` or `${$ref->hash1}{'two'} = 'twosdata';` But personally I definately prefer the first way. `ihb`	[reply] [d/l] [select]
Re: Re: passing object references into subs by Avox (Sexton) on Feb 03, 2003 at 19:51 UTC
ah! dbp++! I did not realize test was already a reference, although I guess that makes sense. I understand how references work, but get hung up on the perl syntax. Many thanks dbp!	[reply]