Can someone explain this?

#1

First I create a proc:

p = proc {
self::X = Class.new
def self.X
X.new
end
}

Then I create a module:

A = Module.new(&p)
puts A.X # => #<A::x:0x9c2e774>

Then i create another module:

B = Module.new(&p)
puts B.X # => #<B::x:0x9c39304>

Everything seems to be ok.

And now to the strange part.

After module B was created A.X started to return instances of B::X

puts A.X # => #<B::x:0x9c38238>

If more modules are created with the same proc method X returns
instances of X from the last created module. If I use “self::X.new”
instead of “X.new” everything works as expected, but I can’t understand
why it works like that with “X.new”. Is this a bug or it’s supposed to
be this way?

#2

On Mar 13, 2011, at 13:10 , Andy B. wrote:

puts A.X # => #<A::x:0x9c2e774>

If more modules are created with the same proc method X returns
instances of X from the last created module. If I use “self::X.new”
instead of “X.new” everything works as expected, but I can’t understand
why it works like that with “X.new”. Is this a bug or it’s supposed to
be this way?

I suggest you send that to ruby-core@

#3

[+ruby-core]

Also note that calling A.module_eval(&p) after your code then changes
both methods to create A instances.

In Ruby 1.9, there is an inline cache for constant references. Dumping
the bytecode for the self.X method
shows this in action:

[3,
[:trace, 8],
4,
[:trace, 1],
[:getinlinecache, :label_11, 0],
[:getconstant, :X],
[:setinlinecache, 0],
:label_11,
[:send, :new, 0, nil, 0, 1],
5,
[:trace, 16],
4,
[:leave]]

If you use self::X.new in A/B.X, this forces constant resolution each
time, and the inline cache is not used:

[3,
[:trace, 8],
4,
[:trace, 1],
[:putself],
[:getconstant, :X],
[:send, :new, 0, nil, 0, 1],
5,
[:trace, 16],
4,
[:leave]]

The proc is compiled only once, and thus so does the method definition.
The two method objects
are in fact distinct, of course.

A.method(:X) == B.method(:X)
=> false

My best guess: the two methods likely share the same instruction
sequence object, and thus the
same inline caches, and the cache is not properly clearing when it
should be.

Michael E.
removed_email_address@domain.invalid
http://carboni.ca/