The only other difference I can find is that if the class doesn't
previouly exist, class_eval on the class will cause an error.
More accurately, if the class doesn't previously exist, trying to access it
(via class_eval) will call const_missing on the parent module (usually
Object, I think?), and it's const_missing that throws the error. (This is
important, I promise.)
The "class" syntax will define the class if it doesn't exist, and reopen it if
it does.
ActiveSupport (and thus, Rails) does autoloading by overriding
const_missing -- which means that when you do this:
Foo::Bar.class_eval do
...
end
If Foo wasn't available, Rails will actually try to autoload a file
called 'foo.rb', somewhere in its autoload path. Then, if that file doesn't
define Foo::Bar, it'll go looking for 'foo/bar.rb', again in that path.
If, instead, you decide to do this:
module Foo
class Bar
...
end
end
In that case, if Foo isn't available, you're defining it right there! Which
means that instead of everything foo.rb presumably defines, you're getting
nothing but what you've actually put between "module Foo" and "end"...
Which is a completely silent error until you decide to actually load foo.rb,
in which case, the only error you'll get is if you thought Foo was a module,
and it was actually a class.
Now, this probably isn't a problem with ActionController, since, being Rails,
that's probably always available.
But if there's going to be autoloading anywhere in the project, I consider it
a best practice to only use "class" or "module" once, where you intend to
originally define the class (or module) in question. If you need to reopen it
to monkeypatch, use _eval.
The same is NOT true of "autoload", by the way. (Seems to be in 1.9, not sure
what versions of 1.8.) That is, if you say:
autoload :Foo, 'foo'
class Foo
This will actually work the way you expect.
There are other problems with autoload, though -- it doesn't support
overridden require commands, nor does it provide any other means of
extensibility beyond modifying the global require path.
