macos NSInteger 设置为 0 但返回 nil

NSInteger set to 0 but returns nil

提问by BytesGuy

I have an NSInteger in my class like so

我的班级中有一个 NSInteger 像这样

In @interface:

在@interface 中：

NSInteger currentRow;    

@property (assign) NSInteger currentRow;

In @implementation:

在@实现中：

@synthesize currentRow;

Doing [self setCurrentRow:0]seems to work fine, but using [self currentRow]just returns null for some reason, not sure why. When using breakpoints I can see that the value for currentRowis 0so it has set fine, but I can't get the value back.

这样做[self setCurrentRow:0]似乎工作正常，但[self currentRow]由于某种原因使用只返回 null，不知道为什么。当使用断点我可以看到，价值currentRow是0所以它有一系列很好，但我不能值回。

回答by paulmelnikow

In Objective-C, it's important that you distinguish between objects and primitive types.

在 Objective-C 中，区分对象和原始类型很重要。

An objectis always stored as a pointer, which is the object's location in memory. A pointer is just a number. With NSLog, you can use %pto see this value. You can display it in the debugger too, like this: print myObject. A pointer is displayed as a hexadecimal number, with a 0xprefix. nilis essentially location zero (0x0000). When you allocate any kind of object, you'll get a pointer which isn't zero. When you assign an object to a variable, you are simply copying the memory address, not duplicating the object. With NSLog, you can use %@to print out an object's description. In the debugger, like this: print-object myObject.

一个对象总是存储作为指针，这是在存储器中的对象的位置。指针只是一个数字。使用NSLog，您可以使用%p查看此值。您可以在调试器中显示它太像这样：print myObject。指针显示为带有0x前缀的十六进制数。nil本质上是位置零 ( 0x0000)。当你分配任何类型的对象时，你会得到一个不为零的指针。当您将对象分配给变量时，您只是复制内存地址，而不是复制对象。使用NSLog，您可以使用%@打印出对象的description. 在调试器，像这样：print-object myObject。

Primitive typeslike NSIntegeraren't objects. Instead of storing a pointer, usually you just store the value. When you assign an NSIntegervariable, you make a copy of the value. You can see the value in the debugger using print. Or like this: NSLog("%ld", (long)currentRow). When you assign a primitive, you copy its value. Don't use %@or print-objectwith primitives — they expect objects.

原始类型如NSInteger不是对象。通常您只存储值而不是存储指针。分配NSInteger变量时，您会复制该值。您可以使用print. 或者像这样：NSLog("%ld", (long)currentRow)。当你分配一个基元时，你复制了它的值。不要使用%@或print-object与原语一起使用——它们需要对象。

(I say "usually you just store the value," because you can make pointers to primitive types, too. In situations like yours however it's not necessary.)

（我说“通常你只存储值”，因为你也可以创建指向原始类型的指针。但在像你这样的情况下，这是没有必要的。）

[self currentRow]returns 0, just like you set it. (Furthermore, because Objective-C guarantees initialization of instance variables, it'll return 0 even if you don'tset it.)

[self currentRow]返回 0，就像你设置的一样。（此外，因为Objective-C 保证实例变量的初始化，即使你不设置它也会返回0。）

The problem is that you're expecting a pointer to an object. How you fix your code depends on how you're using it:

问题是你期待一个指向对象的指针。您如何修复代码取决于您如何使用它：

• If you're using print-object currentRow, change it to print currentRow.
• If you're using NSLog("%@", currentRow), change it to NSLog(%"ld", (long)currentRow).
• If you're using currentRowsomewhere else, where an object is required, change your instance variable and property types to NSNumber *, an object type. Set it with [self setCurrentRow:[NSNumber numberWithInt:0]].

• 如果您正在使用print-object currentRow，请将其更改为print currentRow。
• 如果您正在使用NSLog("%@", currentRow)，请将其更改为NSLog(%"ld", (long)currentRow)。
• 如果您在currentRow其他需要对象的地方使用，请将您的实例变量和属性类型更改NSNumber *为对象类型。用 设置它[self setCurrentRow:[NSNumber numberWithInt:0]]。

回答by secondcitysaint

NSInteger is not object, it's typedef'd to int or something like that. Thus, if you set currentRow to 0 and then try to get it back, null (aka 0) is totally correct value.

NSInteger 不是对象，它是 typedef 到 int 或类似的东西。因此，如果您将 currentRow 设置为 0，然后尝试将其取回，则 null（又名 0）是完全正确的值。

回答by Mattias Wadman

The getter method will be synthesized as - (NSInteger)currentRowso it should work just fine. But how do you check if it works? With NSLog(@"%@", ...)? Than you should use %d.

getter 方法将被合成，- (NSInteger)currentRow因此它应该可以正常工作。但是你如何检查它是否有效？与NSLog(@"%@", ...)？比你应该使用%d.

If you want it to be an object you should use NSNumberand a retainproperty.

如果您希望它是一个对象，您应该使用它NSNumber和一个retain属性。