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Using NSNull

The NSNull class defines a singleton object you use to represent null values in situations where nil is prohibited as a value (typically in a collection object such as an array or a dictionary).

NSNull *nullValue = [NSNull null];

NSArray *arrayWithNull = [NSArray arrayWithObject:nullValue];

NSLog(@"arrayWithNull: %@", arrayWithNull);

// output: "arrayWithNull: (<null>)"

It is important to appreciate that the NSNull instance is semantically different from NO or false—these both represent a logical value; the NSNull instance represents the absence of a value. The NSNull instance is semantically equivalent to nil, however it is also important to appreciate that it is not equal to nil. To test for a null object value, you must therefore make a direct object comparison.

id aValue = [arrayWithNull objectAtIndex:0];

if (aValue == nil) {

    NSLog(@"equals nil");

    }

    else if (aValue == [NSNull null]) {

        NSLog(@"equals NSNull instance");

        if ([aValue isEqual:nil]) {

            NSLog(@"isEqual:nil");

        }

    }

}

// output: "equals NSNull instance"

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