Question

I’m learning Ruby. I’ve got the O’Reilly book, “The Ruby Programming Language,” which states unequivocally:

“Object class implements the hash method to simply return an object’s ID.”

I’ve also seen this assertion in other books: http://my.safaribooksonline.com/book/web-development/ruby/9780321700308/create-classes-that-understand-equality/ch12lev1sec8

But when I run this code, the two lines do not generate the same number:

myObject = Object.new

puts myObject.hash
puts myObject.object_id

So what’s the deal? I’m running Ruby 1.9.3.

Answer 1

The Object implementation hashes the object_id. The value isn’t the object_id, but the object_id is the input to the hash function.

Via https://github.com/ruby/ruby/blob/trunk/object.c#L110

VALUE
rb_obj_hash(VALUE obj)
{
    VALUE oid = rb_obj_id(obj);
    st_index_t h = rb_hash_end(rb_hash_start(NUM2LONG(oid)));
    return LONG2FIX(h);
}