Appending to an existing string

Can I ask why this is important?

I know that this is not a direct answer to your question, but the fact that you are trying to preserve the object ID of a string might indicate that you should look again at what you are trying to do.

You might find, for instance, that relying on the object ID of a string will lead to bugs that are quite hard to track down.

Yet an other way:

s.insert(-1, ' world')

you can also use the following:

s.concat("world")

You can use << to append to a string in-place.

s = "foo"
old_id = s.object_id
s << "bar"
s                      #=> "foobar"
s.object_id == old_id  #=> true