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Programming Perl, 3rd Edition by Jon Orwant, Tom Christiansen, Larry Wall

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Tying Hashes

A class implementing a tied hash should define eight methods. TIEHASH constructs new objects. FETCH and STORE access the key/value pairs. EXISTS reports whether a key is present in the hash, and DELETE removes a key along with its associated value.[2] CLEAR empties the hash by deleting all key/value pairs. FIRSTKEY and NEXTKEY iterate over the key/value pairs when you call keys, values, or each. And as usual, if you want to perform particular actions when the object is deallocated, you may define a DESTROY method. (If this seems like a lot of methods, you didn't read the last section on arrays attentively. In any event, feel free to inherit the default methods from the standard Tie::Hash module, redefining only the interesting ones. Again, Tie::StdHash assumes the implementation is also a hash.)

For example, suppose you want to create a hash where every time you assign a value to a key, instead of overwriting the previous contents, the new value is appended to an array of values. That way when you say:

$h{$k} = "one";
$h{$k} = "two";

It really does:

push @{ $h{$k} }, "one";
push @{ $h{$k} }, "two";

That's not a very complicated idea, so you should be able to use a pretty simple module. Using Tie::StdHash as a base class, it is. Here's a Tie::AppendHash that does just that:

package Tie::AppendHash;
use Tie::Hash;
our @ISA = ("Tie::StdHash");
sub STORE {
    my ($self, $key, $value) = @_;
    push @{$self->{key}}, $value;
}
1;

Hash-Tying Methods

Here's an example of an interesting ...

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