posted by delano

There’s something about Ruby that I’ve wanted to know for a long time: where does the hexadecimal value in #<Object:0x000001009e60f8> come from? Today I finally went looking for the answer.

When in doubt, look at the code

The documentation for Object#to_s tells us that the value is based on the object id: “Returns a string representing obj. The default to_s prints the object‘s class and an encoding of the object id.”

Looking at the source code for to_s, we can see that it’s using sprintf to create the hexadecimal value. From object.c:

/* Ruby 1.9 */
rb_sprintf("#<%s:%p>", cname, (void*)obj);

/* Ruby 1.8 */
snprintf(RSTRING(str)->ptr, len+1, "#<%s:0x%lx>", cname, obj);

You can find this code in the Ruby source code in the file ruby-VERSION/object.c

Now, Ruby’s sprintf doesn’t support %p or %lx but it does support %x. However, there’s obviously more to the story because the values still don’t match up:

obj = Object.new
"#<%s:0x%x>" % [obj.class, obj.object_id]        # => #<Object:0x80813ff4>
obj.to_s                                         # => #<Object:0x101027fe8>

So what do we need to do to the object id to get the real hexadecimal value?

A tiny calculation

If we take a look at the hexadecimal values in plain-old decimal, the answer becomes obvious. Can you see it?

"0x80813ff4".hex                                 # => 2155954164
"0x101027fe8".hex                                # => 4311908328

All we have to do is double it!

And the answer is…

"#<%s:0x%x>" % [obj.class, obj.object_id.abs*2]  # => #<Object:0x101027fe8>

Note: I’ve created a tiny project called Hexoid to handle the minor formatting difference between Ruby 1.8 and 1.9.