在Objective-C中隐藏私人信息的位置[重复]

关于私有实例变量应该在哪里声明以及为什么声明,约定/规则/偏好/命令是什么?

// someClass.h

@interface someClass : NSObject {
@private
// Here in the class declaration?
}
// ...
@end

// someClass.m

@interface someClass () {
// Here in the class extension? This seems to be obj-c's version
// of the C++ Pimpl idiom.
}
@end

@implementation someClass {
// Here in the class definition? BTW, what's the default scope here?
// Seems like it should be @private. I haven't been able to find much
// documentation about this declaration block.
}
// ...
@end

有人能评论这三个部分的适当使用吗?或者指出一个好的网络资源吗?谢谢