What does :: (double colon) mean in Ruby?

Question

What does :: mean in Ruby? E.g. Foo::Bar.

Answer 1

From the Pickaxe:

When a receiver is explicitly specified in a method invocation, it may be separated from the method name using either a period (.) or two colons (::). The only difference between these two forms occurs if the method name starts with an uppercase letter. In this case, Ruby will assume that a receiver::Thing method call is actually an attempt to access a constant called Thing in the receiver unless the method invocation has a parameter list between parentheses.

Answer 2

It's called a scope resolution operator. Basically a fancy way of referencing a class within a namespace. ActiveRecord is the namespace and Base is the class.

Answer 3

It accesses constants in a given class or module. E.g. ActiveRecord::Base is the constant Base defined in the module ActiveRecord.