From: Nikolai Weibull <now@...>
Date: 2012-03-22T00:23:34+09:00
Subject: What’s the best way to check if a feature/class has been loaded?

Hi!

What’s the best way to check if a feature/class has been loaded?

I’ve been using

defined? A::B::C

but that fails to give me the correct result if A is a class and B::C
is defined, but A::B::C isn’t defined, as Ruby will then first resolve
A::B to the top-level constant B and then resolve C within B.

Here’s a test case:

class A
end

module B
end

p defined? A::B::C # => nil

module B
 module C
 end
end

p defined? A::B::C # => "constant"

The only solution I can think of is to do

def feature?(path)
 path.split('::').reduce(Object){ |o, e| begin o.const_get(e, false);
rescue NameError; return nil; end }
end

but that won’t work in 1.8.7, as there’s no second argument to
Module#const_get in 1.8.7.  A solution that works across the 1.8/1.9
divide might be

def feature?(path)
 const = path.split('::').reduce(Object){ |o, e| begin o.const_get(e);
rescue NameError; return nil; end }
 const.name == path ? const : nil
end

and even though that works, I’d rather have something simpler,
something built-in.

An alternative would be to check $LOADED_FEATURES, but, again, the
internals of how require works have changed between 1.8 and 1.9 and,
considering things like file-name extensions and one mustn’t forget
about autoload and its semantics, would also be best left to built-in
functionality.

So, how does one check for the presence of A::B::C without
accidentally referencing top-level constants?