Moose::Cookbook::Basics::Immutable(3pm)

SYNOPSIS

  package Point;
  use Moose;
  has 'x' => ( isa => 'Int', is => 'ro' );
  has 'y' => ( isa => 'Int', is => 'rw' );
  __PACKAGE__->meta->make_immutable;

DESCRIPTION

The Moose metaclass API provides a make_immutable() method. Calling this method does two things to your class. First, it makes it faster. In particular, object construction and destruction are effectively "inlined" in your class, and no longer invoke the meta API.

Second, you can no longer make changes via the metaclass API, such as adding attributes. In practice, this won't be a problem, as you rarely need to do this after first loading the class.

CONCLUSION

We strongly recommend you make your classes immutable. It makes your code much faster, with a small compile-time cost. This will be especially noticeable when creating many objects.