NAME
Hash::DefaultValue - create a hash where the default value ain't undef
SYNOPSIS
use 5.010;
use Hash::DefaultValue;
tie my %hash, 'Hash::DefaultValue', 42;
say $hash{the_answer}; # says 42
DESCRIPTION
Normally, if you try fetching a value from a hash where the key does not
exist, you get undef.
my %hash;
if (defined $hash{foobar}) {
say "this will not happen!";
}
Hash::DefaultValue allows you to choose another value instead of undef.
It tried to avoid changing any other part of the hash's behaviour. For
example, it doesn't automatically create any hash keys that Perl
wouldn't normally autovivify.
tie my %hash, 'Hash::DefaultValue', 42;
say $hash{the_answer}; # says 42
my $exists = exists $hash{the_answer}; # false
say keys %hash; # says nothing
And of course you can still store explicit values in the hash, as you'd
expect:
tie my %hash, 'Hash::DefaultValue', 42;
$hash{monkey} = 'Bobo';
say $hash{the_answer}; # says 42
say $hash{monkey}; # says "Bobo"
Delete operations on the hash are vaguely interesting:
tie my %hash, 'Hash::DefaultValue', 42;
$hash{monkey} = 'Bobo';
my $the_answer = delete $hash{the_answer}; # undef
my $monkey = delete $hash{monkey}; # "Bobo"
Allowed Default Values
Any non-reference scalar can be used as a default value.
Coderefs can be used too, in which case when a default value is being
fetched the coderef will be evaluated (in scalar context) and the return
value used as the default. The coderef will have a reference to the tied
hash, and the key being fetched passed as arguments. Additionally, the
key will be available in $_ which often makes for nicer looking code.
tie my %hash, 'Hash::DefaultValue', sub { $_ + 10 };
say $hash{32}; # says 42
say $hash{monkey}; # says 10
Other references are disallowed, which provides a handy extensibility
point in the future. If you want to use some other reference, then wrap
it in a coderef.
tie my %hash, 'Hash::DefaultValue', sub { \@foo };
Alias
The aliased module allows you to define aliases for class names, and
works great for tie implementations.
use aliased 'Hash::DefaultValue' => 'HDV';
tie my %hash, HDV, 42;
BUGS
Please report any bugs to
<
http://rt.cpan.org/Dist/Display.html?Queue=Hash-DefaultValue>.
SEE ALSO
Hash::Missing is a subclass of this module.
Hash::WithDefaults allows you to default particular keys by providing a
template hashref.
AUTHOR
Toby Inkster <
[email protected]>.
COPYRIGHT AND LICENCE
This software is copyright (c) 2012 by Toby Inkster.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.
DISCLAIMER OF WARRANTIES
THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
