在ObjectiveC中，是否有必要为只读属性声明强vs copy ?

In Objective-C, it's common to declare NSString/NSArray/NSDictionary as copy, is it necessary to do that for a readonly property or there is no difference? If an NSString is readonly, it will never be set, so declaring it strong or copy will have the same effect right?In Objective-C, it's common to declare NSString