By default, Ruby will take your constant to be publicly available. However, it’s important to note these should not be visible by anyone else but the module in which it was defined.

Let’s run some examples on irb:

class User
 NUMBER = 100
end

and then running it:

User::NUMBER # => 100

as expected, we are able to access the value of the constant NUMBER.

Still, as I’ve said before, you should often not make them public. For that, we must use private constants.

With that in mind, you may be wondering if it’d be possible to simply declare the constant to be private. Well, that’s unfortunately not the case:

class User
  private
  NUMBER = 100
end

running it:

User::NUMBER # => 100

Bummer. It’s still accessible… So, what do we do?

That’s where private_constant comes up:

class User
  NUMBER = 100
  private_constant :NUMBER
end

Let’s test if it’s still accessible:

User::NUMBER
# => NameError: private constant User::NUMBER referenced

Nope. We’ve just made it that constant private! That’s it.

You may be wondering when you should use this. As a rule of thumb, this is a great implementation whenever it’s necessary to ensure a constant is an implementation detail and only meant to be used internally.