Difference between Foo = Class.new and class Foo


#1

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


#2

2008/10/7 Brandon D. 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


#3

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 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


#4

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


#5

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 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.

#6

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