String subclass method returns subclass - bug or feature?


#1

Could you please explain me why String instance methods return subclass,
and MyStr#to_s returns String, not the receiver as specified?

here is the test code

$ cat tStringSubClass.rb
class MyStr < String
def rest
self[1…-1]
end
end
ms = MyStr.new(‘ABC’)
puts
'I got ’ + ms.class.name,
‘Expect String, get:’,
ms.rest.class,
ms.downcase.class,
ms.succ.class,
ms[1…-1].class,
ms.split(//).map{ |s| s.class },
‘Expect MyStr, get:’,
ms.to_s.class,
ms.to_str.class

and here is the result

$ ruby tStringSubClass.rb
I got MyStr
Expect String, get:
MyStr
MyStr
MyStr
MyStr
MyStr
MyStr
MyStr
Expect MyStr, get:
String
String

thanks
brs
Sergey


#2

“S.Volkov” removed_email_address@domain.invalid writes:

Could you please explain me why String instance methods return subclass,
and MyStr#to_s returns String, not the receiver as specified?

String (and Array) methods that return new strings (arrays) tend to
return instances of the receiver’s class rather than String. This
often makes subclassing a bit less painful, although a couple of pies
have been flung over it too:

That last thread is probably what led to the following Changelog
entry, which explains the behaviour of MyStr#to_s you saw.

Mon Feb 10 10:14:26 2003 Yukihiro M. removed_email_address@domain.invalid

    * array.c (rb_ary_to_a): return value should be an Array if the
  receiver is an instance of subclass of Array.

    * string.c (rb_str_to_s): return value should be a String if the
  receiver is an instance of subclass of String.

#3

Thanks for references,
I understand that it was discussed already, and decision was made;
imho: it’s irregular and not OO, but who cares about formal, theoretical
correctness, ‘be practical’ criteria rules!

Regarding ‘to_s, to_str returning String’: hope documentation will be
fixed;

regards
Sergey

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