objective c - NSInteger set to 0 but returns nil

Question

I have an NSInteger in my class like so

In @interface:

NSInteger currentRow;    

@property (assign) NSInteger currentRow;

In @implementation:

@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 currentRow is 0 so it has set fine, but I can't get the value back.

Answer 1

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

An object is always stored as a pointer, which is the object's location in memory. A pointer is just a number. With NSLog, you can use %p to 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 0x prefix. nil is 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.

Primitive types like NSInteger aren't objects. Instead of storing a pointer, usually you just store the value. When you assign an NSInteger variable, 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-object with primitives — they expect objects.

(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.)

[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't set it.)

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

• 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 currentRow somewhere 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]].