How to get all class names in a namespace in Ruby?

Foo.constants

returns all constants in Foo. This includes, but is not limited to, classnames. If you want only class names, you can use

Foo.constants.select {|c| Foo.const_get(c).is_a? Class}

If you want class and module names, you can use is_a? Module instead of is_a? Class.

If, instead of the names of the constants, you want the classes themselves, you could do it like this:

Foo.constants.map(&Foo.method(:const_get)).grep(Class)

In short no. However, you can show all classes that have been loaded. So first you have to load all classfiles in the namespace:

Dir["#{File.dirname(__FILE__)}/lib/foo/*.rb"].each {|file| load file}

then you can use a method like Jörg W Mittag's to list the classes

Foo.constants.map(&Foo.method(:const_get)).grep(Class)