From: Robert Klemme <bob.news@...>
Date: 2005-07-15T07:10:51+09:00
Subject: Re: Difference between Class.new and class

Levin Alexander <levin@grundeis.net> wrote:
> Hi,
>
> why do class definitions with the keyword "class" return nil and not
> the class itself?

Because the last expression in the block is returned:

>> class Foo; self end
=> Foo
>> class Bar; 123 end
=> 123

Method definitions ("def ...") return nil.

>    >> class Foo
>    >>   def foo
>    >>     puts "in Foo"
>    >>   end
>    >> end
>    => nil
>
> and what is the difference to:
>
>    >> Bar = Class.new do
>    >>   define_method(:bar) do
>    >>     puts "in Bar"
>    >>   end
>    >> end
>    => Bar

The former is the standard syntax for defining classes.  The latter can be 
used to create anonymous classes.

>> Irks = Class.new
=> Irks
>> ui = Class.new
=> #<Class:0x10189d98>

A disadvantage of the latter approach is that you cannot define methods that 
receive a block.  There is no equivalent (yet) for "def foo(&b) b.call(1) 
end" with define_method, because you cannot do define_method(:foo) {|&b| 
b.call(1)}.  Also, you cannot have default values for method arguments with 
define_value.  The bottom line is: use the block form only if you need 
anonymous classes or have to otherwise.  HTH

Kind regards

    robert