@property int age;
@synthesize age = _age;

Person *p =[ [Person alloc] init]
  p.age = 1;
  NSLog(@"_age = %d",p.age);

@property int age;
@synthesize age = _age;

Person *p =[ [Person alloc] init]
  p.age = 1;
  NSLog(@"_age = %d",p.age);