gistfile1.m

#import <Foundation/Foundation.h>

@interface ClassA : NSObject
@end

@implementation ClassA
@end

@interface ClassB : NSObject
@end

@implementation ClassB
@end

int main(void) {
  {
    ClassA *objA = [[ClassB alloc] init]; // OK
    [objA release];
  }

  {
    ClassA *objA = [[ClassA alloc] init];
    ClassB *objB = objA; // OK
    [objB release];
  }

  return 0;
}