NSNumber 的副本没有分配新内存
我正在为自定义 A 类实现 copyWithZone 方法，其中 NSNumber 指针被声明为（保留）属性
@class A <NSCopying>
{
NSNumber *num;
}
@property (nonatomic, retain) NSNumber *num; // synthesized in .m file
-(id) copyWithZone:(NSZone*) zone {
A *new = [[A alloc] init];
new.num = [num copy];
return new;
}
当我调试时，我总是发现 new.num 与 self.num 的地址相同。
即使我使用
new.num = [NSNumber numberWithFloat: [num floatValue]];
我仍然得到相同的地址。最终我还是得用它
new.num = [[[NSNumber alloc] initWithFloat:[num floatValue]] autorelease]
来达到我想要的结果。我只是想知道为什么 NSNumber 遵守但在复制时不返回新的内存地址？
谢谢利奥
I am implementing a copyWithZone method for a custom A class, in which a NSNumber pointer was declared as (retain) property
@class A <NSCopying>
{
NSNumber *num;
}
@property (nonatomic, retain) NSNumber *num; // synthesized in .m file
-(id) copyWithZone:(NSZone*) zone {
A *new = [[A alloc] init];
new.num = [num copy];
return new;
}
When I debug, I always find new.num is the same address as the self.num.
Even if I use
new.num = [NSNumber numberWithFloat: [num floatValue]];
I still get the same address. In the end, I have to use
new.num = [[[NSNumber alloc] initWithFloat:[num floatValue]] autorelease]
to achieve the result I want. I am just wondering why NSNumber complies to but does not return a new memory address when copied?
Thanks
Leo
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NSNumber 是不可变的。制作副本是没有意义的，因此，当调用复制时，框架只会返回 self 。
如果一个类实现了 NSCopying，您应该将该属性标记为
copy（而不是retain）。不可变类 (NSString) 上的-copy只会返回对对象的引用（带有增加的保留计数）。如果传递一个可变实例，它将被复制到一个不可变实例。这可以防止外部人员背后更改对象的状态。NSNumber is immutable. Making a copy is pointless and, thus, the frameworks just return self when copy is invoked.
If a class implements NSCopying, you should mark the property as
copy(notretain).-copyon immutable classes (NSString) will simply return a reference to the object (w/a bumped retain count). If passed a mutable instance, it'll be copied to an immutable instance. This prevents an external party from changing the state behind your object's back.NSNumber 不仅是不可变的 - 对于低值，它也是 Flyweight。
Not only is NSNumber immutable - for low values it as also a Flyweight.
NSNumber不可变，因此无需强制进行物理复制。NSNumberisn't mutable, so there is no need to force physical copying.在实现 NSCopying 协议时，您应该使用 [[A alloc] initWithZone:zone] 。
正如其他人所说， NSNumber 是不可变的，因此返回相同的对象。
You should be using [[A alloc] initWithZone:zone] when implementing the NSCopying protocol.
As others have stated though, NSNumber is immutable and so returns the same object.