使用@property和'copy'属性分配NSMutableArray(assign NSMutableArray with @property and 'copy' attribute)

 我正在关注官方教程你的第二个iOS应用程序：Storyboard ，它告诉我像这样声明一个属性masterBirdSightingList （只是一个特定的例子，不需要知道上下文）： 
 
@property (nonatomic, copy) NSMutableArray *masterBirdSightingList;
 
 请注意，有一个属性副本 。 然后合成这个属性： 
 
@synthesize masterBirdSightingList = _masterBirdSightingList;
 
 接下来有一个让我困惑的init方法： 
 
- (void)initializeDefaultDataList {
NSMutableArray *sightingList = [[NSMutableArray alloc] init];
self.masterBirdSightingList = sightingList;
[self addBirdSightingWithName:@"Pigeon" location:@"Everywhere"];
}
 
 绝对是为空间分配了sightingList，然后将它分配给masterBirdSightingList属性。 但是，该属性具有复制属性。 这意味着实例变量_masterBirdSightingList将被分配给另一个空间以保存来自sightingList的东西。 为什么？ 为什么不直接为属性分配空间，如下所示： 
 
self.masterBirdSightingList = [[NSMutableArray alloc] init];

I'm following an official tutorial Your second iOS App:Storyboard and it told me to declare a property masterBirdSightingList like this(just a specific example and not necessary to know the context) : 
 
@property (nonatomic, copy) NSMutableArray *masterBirdSightingList;
 
Note that there's an attribute copy. and then synthesize this property : 
 
@synthesize masterBirdSightingList = _masterBirdSightingList;
 
And next there's one init method which made me confused :
 
- (void)initializeDefaultDataList {
NSMutableArray *sightingList = [[NSMutableArray alloc] init];
self.masterBirdSightingList = sightingList;
[self addBirdSightingWithName:@"Pigeon" location:@"Everywhere"];
}
 
Definitely sightingList is allocated for spaces and then it's assigned to the masterBirdSightingList property. The property has a copy attribute, though. it means the instance variable _masterBirdSightingList would be allocated for another space to preserve stuffs from sightingList. Why? Why not directly allocate space for the property like this :
 
self.masterBirdSightingList = [[NSMutableArray alloc] init];

原文：https://stackoverflow.com/questions/10724866