Question

I have a Perl class that contains a hash instance variable for storing other objects. I would like to have a method to print the number of elements in the hash but I am getting the following error message on the line return keys($self->{'_things'});

Type of arg 1 to keys must be hash (not hash element)

package MyClass;
use strict;

sub new {
    my ($class) = @_;
    my $self = {
        _things => undef
    };
    $self->{'_things'} = ();
    bless $self, $class;
    return $self;
}

sub get_count {
    my ( $self ) = @_;
    return keys($self->{'_things'});
}

Answer 1

Use

return scalar(keys(%{$self->{'_things'}}));

$self->{'_things'} is just a reference to a hash, and keys() expects a hash as its argument—so, you have to dereference it first by wrapping it in %{…}. Finally, since you want to count items, you have to make sure the return value of keys() (which is a list) is interpreted in scalar context, by wrapping it in scalar(…).