Perl Hacks

Hack #46. Make Methods Really Private

Enforce encapsulation with a little more flair.

Perl's object orientation is powerful in many ways, allowing the creation and emulation of almost any kind of object or class system. It's also very permissive, enforcing no access control by default. Any code can poke and prod methods and parents into any class at any time and can call even ostensibly private methods regardless of the intent of the code's original author.

By convention, the Perl community considers methods with a leading underscore as private methods that you shouldn't override or call outside of the class or rely on any specific semantics or workings. That's usually a good policy, but there's little enforcement and it's only a convention. It's still possible to call the wrong method accidentally or even on purpose.

Fortunately, there are better (or at least scarier) ways to hide methods.

The Hack

One easy way to manipulate subroutines and methods at compile time is with subroutine attributes [Hack #45]. The Class::HideMethods module adds an attribute to methods named Hide that makes them unavailable and mostly uncallable from outside the program:

package Class::HideMethods; use strict; use warnings; use Attribute::Handlers; my %prefixes; sub import { my ($self, $ref) = @_; my $package = caller( ); $prefixes{ $package } = $ref; } sub gen_prefix { my $invalid_chars = "\\0\\r\\n\\f\\b"; my $prefix; for ( 1 .. 5 ) { my $char_pos = int( rand( length( $invalid_chars ) ) ); $prefix ...