# Difference between Foo = Class.new and class Foo

**URL:** <https://www.ruby-forum.com/t/difference-between-foo-class-new-and-class-foo/150350>\
**Category:** Ruby\
**Created:** [October 7, 2008, 5:31pm UTC](https://www.ruby-forum.com/t/difference-between-foo-class-new-and-class-foo/150350 "2008-10-07T17:31:54Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![bdimcheff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/779978/32.png) [@bdimcheff](https://www.ruby-forum.com/u/bdimcheff)\
**Post date:** [October 7, 2008, 5:31pm UTC](https://www.ruby-forum.com/t/difference-between-foo-class-new-and-class-foo/150350/1 "2008-10-07T17:31:54Z")

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I’m playing around with classes and discovered that

class Foo  
…  
end

and

Foo = Class.new do  
…  
end

seem to behave slightly differently. I assumed that “class Foo” was  
just a shortcut that got translated into “Foo = Class.new”, but when I  
muck with class in the following way:

class Class  
class \<\< self  
def new  
puts “new called”  
super  
end  
end  
end

and then do “Foo = Class.new”, I get “new called” printed to stdout.  
If I do “class Foo…” I get nothing. So it seems that the “class”  
keyword does not end up calling new on Class when you define a new  
class. Does anybody know what’s going on here? Is something else  
called instead?

Thanks,  
Brandon

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<div class="post-metadata">

**Author:** ![bdimcheff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/779978/32.png) [@bdimcheff](https://www.ruby-forum.com/u/bdimcheff)\
**Post date:** [October 7, 2008, 6:00pm UTC](https://www.ruby-forum.com/t/difference-between-foo-class-new-and-class-foo/150350/2 "2008-10-07T18:00:52Z")

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2008/10/7 Brandon D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> and then do “Foo = Class.new”, I get “new called” printed to stdout. If I  
> do “class Foo…” I get nothing. So it seems that the “class” keyword does  
> not end up calling new on Class when you define a new class. Does anybody  
> know what’s going on here? Is something else called instead?

Looks like there is a shortcut:

16:55:57 bas$ ruby -e ‘set\_trace\_func lambda {|\*a|p a}; class Foo;end’  
[“line”, “-e”, 1, nil, #Binding:0x1002f0d4, false]  
[“c-call”, “-e”, 1, :inherited, #Binding:0x1002ef80, Class]  
[“c-return”, “-e”, 1, :inherited, #Binding:0x1002ef44, Class]  
[“class”, “-e”, 1, nil, #Binding:0x1002ed64, false]  
[“end”, “-e”, 1, nil, nil, false]  
16:56:13 bas$ ruby -e ‘set\_trace\_func lambda {|\*a|p a}; Foo=Class.new’  
[“line”, “-e”, 1, nil, #Binding:0x1002f0e8, false]  
[“c-call”, “-e”, 1, :new, #Binding:0x1002f0ac, Class]  
[“c-call”, “-e”, 1, :initialize, #Binding:0x1002eeb8, Class]  
[“c-call”, “-e”, 1, :inherited, #Binding:0x1002edb4, Class]  
[“c-return”, “-e”, 1, :inherited, #Binding:0x1002ed78, Class]  
[“c-return”, “-e”, 1, :initialize, #Binding:0x1002ec88, Class]  
[“c-return”, “-e”, 1, :new, #Binding:0x1002eb98, Class]  
16:56:44 bas$

If you need hooks for class creation look closely at “:inherited” above.

Kind regards

robert

---

<div class="post-metadata">

**Author:** ![bdimcheff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/779978/32.png) [@bdimcheff](https://www.ruby-forum.com/u/bdimcheff)\
**Post date:** [October 7, 2008, 6:01pm UTC](https://www.ruby-forum.com/t/difference-between-foo-class-new-and-class-foo/150350/3 "2008-10-07T18:01:27Z")

</div>

Hi,

In message “Re: Difference between Foo = Class.new and class Foo”  
on Tue, 7 Oct 2008 23:51:03 +0900, Brandon D.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

|I’m playing around with classes and discovered that  
|  
|class Foo  
| …  
|end  
|  
|and  
|  
|Foo = Class.new do  
| …  
|end  
|  
|seem to behave slightly differently. I assumed that “class Foo” was  
|just a shortcut that got translated into “Foo = Class.new”, but when I  
|muck with class in the following way:  
|  
|class Class  
| class \<\< self  
| def new  
| puts “new called”  
| super  
| end  
| end  
|end  
|  
|and then do “Foo = Class.new”, I get “new called” printed to stdout.  
|If I do “class Foo…” I get nothing. So it seems that the “class”  
|keyword does not end up calling new on Class when you define a new  
|class. Does anybody know what’s going on here? Is something else  
|called instead?

I think you’re comparing the different code. See the following code:

class Foo  
class \<\< self  
def new  
puts “foo new called”  
super  
end  
end  
end

Foo.new

Bar = Class.new do  
class \<\< self  
def new  
puts “bar new called”  
super  
end  
end  
end

Bar.new

---

<div class="post-metadata">

**Author:** ![bdimcheff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/779978/32.png) [@bdimcheff](https://www.ruby-forum.com/u/bdimcheff)\
**Post date:** [October 7, 2008, 6:20pm UTC](https://www.ruby-forum.com/t/difference-between-foo-class-new-and-class-foo/150350/4 "2008-10-07T18:20:52Z")

</div>

hmm, interesting. I’m definitely going to have to use set\_trace\_func  
more often. Do you know if there’s any way to tell which c function  
is being called for the c-calls? Like how does the ruby method  
“inherited” map to rb\_… in MRI?

Thanks,  
Brandon

---

<div class="post-metadata">

**Author:** ![bdimcheff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/779978/32.png) [@bdimcheff](https://www.ruby-forum.com/u/bdimcheff)\
**Post date:** [October 7, 2008, 6:27pm UTC](https://www.ruby-forum.com/t/difference-between-foo-class-new-and-class-foo/150350/5 "2008-10-07T18:27:57Z")

</div>

Hi,

In message “Re: Difference between Foo = Class.new and class Foo”  
on Wed, 8 Oct 2008 00:32:41 +0900, Brandon D.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

|I don’t think I’m doing conceptually different things below. I’m  
|looking at creating and defining classes, not creating objects from  
|those classes. I’m attempting to create a class called Foo in both  
|cases. The first time, I use the class keyword. The second time, I  
|call Class.new. It seems that using the class keyword doesn’t call  
|Class’s metaclass’s “new” method, while Class.new (obviously) does.

Now I know what you meant. Literals and expressions do not go through  
“new” method as a general rule.

```
          matz.

```

---

<div class="post-metadata">

**Author:** ![bdimcheff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/779978/32.png) [@bdimcheff](https://www.ruby-forum.com/u/bdimcheff)\
**Post date:** [October 7, 2008, 6:04pm UTC](https://www.ruby-forum.com/t/difference-between-foo-class-new-and-class-foo/150350/6 "2008-10-07T18:04:23Z")

</div>

matz,

I don’t think I’m doing conceptually different things below. I’m  
looking at creating and defining classes, not creating objects from  
those classes. I’m attempting to create a class called Foo in both  
cases. The first time, I use the class keyword. The second time, I  
call Class.new. It seems that using the class keyword doesn’t call  
Class’s metaclass’s “new” method, while Class.new (obviously) does.

Thanks,  
Brandon