Objective-C中NSNumber的不同值long

Question

I have a variable which is a NSNumber of type long. It should hold the value -1. When I log it to the Console it shows the expected value -1.

NSLog(@"myVariable %@", self.myVariable);

But the following expression in the if-clause is false.

    if (myVariable == [NSNumber numberWithInt:-1]) {
    ...
    }

The debugger shows the value 72057594037927935. Does anybody know what's wrong with it?

Thanks!

Answer 1

When you compare NSNumber to other objects, you have two options that do different things:

• You can use == to check if two objects represent the same exact object instance , or
• You can use isEqual: method to check if two objects represent the same value .

In your case the safest approach would be to use the second alternative

if ([myVariable isEqual:[NSNumber numberWithLong:-1]]) {
    ...
}

The == approach may or may not work, depending on the way in which you produced myVariable .

Answer 2

NSObject has a method: (NSString *)description This method will define which is printed out when you put object on NSLog method.

• NSNumber inherits NSObject and description method is implemented in the way that primitive value will be printed out, and that's why you saw expected value using NSLog(@"myVariable %@", self.myVariable);

• Operator "==" will compare 2 objects in this case (compare both pointer and value) ==> it will return false in your case

• If you want to compare primitive value of 2 NSNumbers, use following method instead (BOOL)isEqualToNumber:(NSNumber *)number;