iVar not recognized if declaring my own getter & setter [duplicate]

Answer 1

You need to declare the instance variable in the @interface if you're going to use your own setter/getter:

@interface MyClass : NSObject
{
    CGFloat _scale;
}
...
@end

EDIT (after OP question):

Your particular implementation certainly makes a difference (they aren't simple getter/setter methods as they do something as a side-effect). However I think it's always a good idea to declare your variables and absolutely necessary if you want to target 32-bit OSX, as the runtime doesn't support auto-declared instance variables.

Answer 2

If you provide a full implementation (getter and setter) the compiler assumes you won't need an ivar so if you still want one, you have to create one yourself. Either make an ivar or let @synthesize do it for you. The end result is the same (an ivar is created); the only difference is in the first case you can actually call it whatever you want.

Slightly different but related I often have a

@property (nonatomic,readonly) SomeThing* something;

and provide a method

- (Something*) something { return xxxxxx; }

for cases where you have a method that returns something but having a property is a shortcut.