Constant Visibility

It is less common, but similar to methods, constants have a visibility attached to them. You have the choice between private and public, and you can also mark a constant deprecated!

Like with methods, the default visibility of a constant is public. Unlike methods, which have a lot of associated methods for metaprogramming, working with constants is easier:

Module.methods.grep /const/
=> [:const_get, :constants, :const_defined?, :const_set, :private_constant,
:public_constant, :deprecate_constant, :const_missing]

Module.private_methods.grep /const/
=> [:remove_const]

Besides some minor idiosyncrasies (like the visibility modifiers, which where introduced at a later time, refer to constants with constant, while the others just use const), their usage is straight forward.

Effects of Visibility Modifications

You must create a constant before you can modify its visibility. When a constant exists, you can use the class method private_constant or deprecate_constant to alter its behaviour. You can change private visibility back to the original state using the public_constant method. You cannot "undeprecate" constants.


This is the default behavior, the constant can be referenced from anywhere and shows up in the constants list:

module Namespace
  module Public

Namespace::Public # => Namespace::Public

Namespace.const_defined?(:Public) # => true
Namespace.constants.include?(:Public) # => true


The constant cannot referenced via the :: operator and does not show up in the constants list. They can still be accessed from within the namespace and via const_get:

module Namespace
  module Private

  private_constant :Private

Namespace::Private # NameError: private constant Namespace::Private referenced

module Namespace
  Private # => Namespace::Private

Namespace.const_defined?(:Private) # => true
Namespace.constants.include?(:Private) # => false

Namespace.const_get(:Private) # => Namespace::Private


This is an additional property, which is also stored in a constant's visibility flag. Whenever you reference it, it will output a ($VERBOSE level independent) warning:

module Namespace
  module Deprecated

  deprecate_constant :Deprecated

Namespace::Deprecated # => Namespace::Deprecated
# warning: constant Namespace::Deprecated is deprecated

Namespace.const_defined? :Deprecated # => true
Namespace.constants.include? :Deprecated # => true

More about Constants

More Idiosyncratic Ruby