NSString subclass responsibility

[Boris D.]

Hello,
In NSString.m, -length is defined as follows:

- (NSUInteger) length
{
  [self subclassResponsibility: _cmd];
  return 0;
}

My code links with the gnustep-base library but is not compiled from
Objective-C source. Am I supposed to use a subclass of NSString instead
of NSString directly? Which class provides this implementation?