hoge.m

// clang hoge.m -o hoge -framework Foundation -fobjc-arc

#import <Foundation/Foundation.h>

@interface A : NSObject {
  double dummy;
}
@property (strong, nonatomic, setter=setDummyNumber:, getter=dummyNumber) NSNumber *number;
@end

@implementation A
- (void)setDummyNumber:(NSNumber *)number {
  dummy = [number doubleValue];
}

- (NSNumber *)dummyNumber {
  return [NSNumber numberWithDouble:dummy];
}
@end

int main() {
  A *a = [[A alloc] init];
  a.number = @123;
  NSLog(@"a.number = %@", a.number);
}