Where is my Class?

Ruby
1

Hi all,

There isn’t any way to have a class name starting with lowercase letter.

I was trying it as

class a
end

SyntaxError: compile error

class/module name must be CONSTANT

so then i tried

a = Class.new

#Class:0x2de1f00

this shows that “a” is now a reference to the class #Class:0x2de1f00
the Class object

and
B = Class.new

B

that works absolutely fine.

The point is if we try to assign as

B=a
it gives

warning: already intialized constant B … thats fine

but
and now if i check a as

a

B

so where is the original class of a that was #Class:0x2de1f00 … and
how
come the assignment happens from RIGHT-to-LEFT ?

any idea ?

2

assignment happened LEFT-to-RIGHT

3

sur max schrieb:

warning: already intialized constant B … thats fine

any idea ?

sur max, you still have the same class, accessible via both a and B:

a = Class.new # => #Class:0x310b640
a.object_id # => 25713440

B = Class.new # => B
B.object_id # => 25706400

B = a # warning: already initialized constant B
# => B
a # => B
a.object_id # => 25713440
B.object_id # => 25713440

The reason is that Ruby uses the name of the first constant a class
object is assigned to as the class’ name:

X = Class.new # => X
Y = X # => X
Y # => X

In your case, you created an “anonymous” class without assigning it to a
constant, so it doesn’t have a name yet. After assigning this anonymous
class to the constant B, you get the warning, but nethertheless the
constant is changed to reference the previous anonymous class. After the
assignment, the class isn’t anonymous anymore, it has got the name “B”.

This way you can create multiple classes with the same name:

c1 = Class.new # => #Class:0x30f60c4
c2 = Class.new # => #Class:0x30f3aa4

C = c1 # => C
C = c2 # warning: already initialized constant C
# => C

c1 # => C
c2 # => C

They still are distinct classes, though.

c1.object_id # => 25669730
c2.object_id # => 25664850

Regards,
Pit

4

The first time an anonymous class is assigned to a constant, its name
changes to be the constant’s. You can check that everything works as you
expected by doing

puts a.object_id
puts b.object_id

5

Hi Pit,

from your explanation and my perception too it seems that we are not
able to
write anything in the constants once they defined.
Look at this then…

B=Class.new # B

B.module_eval{define_method :foo do; puts “foo here”; end} #
#Proc:0x02deeb18@:3(irb)

B.instance_methods.include?(“foo”) # true

B.id # 24100884

a=Class.new # Class:0x2de7978

a.module_eval{define_method :bar do; puts “bar here”; end}
#Proc:0x02de2920@:7(irb)

a.id # 24067260

a.instance_methods.include?(“foo”) # false

a.instance_methods.include?(“bar”) # true

B=a # warning: already initialized constant B
=> B

a.instance_methods.include?(“bar”) # true

a.instance_methods.include?(“foo”) # false

B.instance_methods.include?(“foo”) # false

B.instance_methods.include?(“bar”) # true

B.id # 24067260

B.id==a.id # true

this means that the Class B is overwritten by the Class referenced by
“a”
so it is the case of changing a constant. Is that possible ?

6

Hi Pit,
Thanks a lot for explaining.

7

sur max schrieb:

(…)
B=a # warning: already initialized constant B
(…)
this means that the Class B is overwritten by the Class referenced by “a”
so it is the case of changing a constant. Is that possible ?

No, no objects are overwritten in Ruby. An assignment like

var = obj

just makes the object on the right (obj) accessible via the name on the
left (var). The line

B = a

makes the object referenced by a (the anonymous class) accessible via
the name B. After this, a and B are different names for the same object.
The previous object referenced by the name B is still around (until the
garbage collector reclaims it) and not changed at all.

In Ruby, constants, instance, and local variables are only names for
objects. Constants have the additional property that they are intended
to be assigned to only once. If you re-assign a constant, you get a
warning, but the assignment happens nonetheless. Try this with something
different than anonymous classes and it might be easier to understand.

There have been many discussions about variable names and objects on
this mailing list. You can also look at

http://onestepback.org/index.cgi/Tech/Ruby/Shoeboxes.rdoc

Regards,
Pit