Question

If I have a custom Ruby class representing some string type, as in

class MyString
end

Which functions should I implement in order to make the following use cases possible:

1. Passing a Ruby string whenever a MyString is expected
2. Passing a MyString whenever a Ruby string is expected
3. Comparing a Ruby string with a MyString value (it shouldn’t matter whether I use s == t or t == s).

I saw various interesting functions like to_s, cmp, == and eq already, but it’s not clear to me when each of them is called.

My concrete use case is that I’m writing a Ruby extension using the C API which exposes functions taking (and returning) values of a custom string type (QString, to be precise) which my extension also registers. However, I’d like to make those custom strings behave as intuitive as possible. Unfortunately I can’t just return Ruby strings from my C code since it should be possible to call Qt methods on the strings.

Answer 1

There are at least three approaches:

1. class MyString < String; ...; end
2. Define #to_s
3. Define #to_str

Doing both #2 and #3 will make the object act very much like a real String even if it isn’t a subclass.

#to_s is an explicit converter, meaning it must appear in Ruby code to work.

#to_str is an implicit converter, meaning the Ruby interpreter will attempt to call it when it wants a String but is given something else.

Update:

Here is an example of some fun you can have with to_str:

begin
  open 1, 'r'
rescue TypeError  => e
  p e
end
class Fixnum
  def to_str; to_s; end
end
open 1, 'r'

When run, the first open fails with TypeError but the second proceeds to looking for 1.

#<TypeError: can't convert Fixnum into String>
fun.rb:9:in `initialize': No such file or directory - 1 (Errno::ENOENT)
    from fun.rb:9:in `open'