Question

After parsing the JSON response from a webservice using NSJSONSerialization, I use +isKindOfClass: to make sure the server returned the kind of data I expect. Using this method, I ran into some weird behaviour, which I’ll illustrate using an example.

Consider the following objects:

// Definitions
NSDictionary *son = @{ @"firstname" : @"James", @"lastname" : @"Appleseed" };
NSDictionary *daughter = @{ @"firstname" : @"Susan", @"lastname" : @"Appleseed"};
NSArray *children = @[son, daughter];
NSDictionary *father = @{ @"firstname" : @"John", @"lastname" : @"Appleseed" };
NSDictionary *family = @{@"children" : children, @"father" : father};
NSDictionary *pedigree = @{@"family" : family };

Those objects represent deserialized JSON returned from a server. Now if I want to use the array of children to calculate how much children there are using NSArray’s -count, I need to make sure the children object is an NSArray. If the children object for example happens to be a string, while the app expects an array, it’ll crash because strings don’t implement a count method. Consider the following code sequence which implements the described check:

// First test
id _family = [pedigree objectForKey:@"family"];
if ([_family isKindOfClass:[NSDictionary class]])
{
    NSDictionary *_family = (NSDictionary *)_family;
    id _children = [_family objectForKey:@"children"];

    NSLog(@"Children: %@", _children);
    NSLog(@"Children classname: %@", NSStringFromClass(children.class));

    if ([_children isKindOfClass:[NSArray class]]) {
        NSLog(@"Children is an NSArray");
    } else {
        NSLog(@"Children is not an NSArray");
    }
} else {
    NSLog(@"Family is not an NSDictionary");
}

After running this code, the console outputs the following:

Children: (null)
Children classname: __NSArrayI
Children is not an NSArray

The console output appears to be extremely remarkable and even contradictory. How could children not be an NSArray while its classname is __NSArrayI?

After a bit of debugging, I found that there are two ways to solve this problem:

1. remove this line NSDictionary *_family = (NSDictionary *)_family;
2. use a different name than _family for the casted variable

How could this behaviour be explained?

Answer 1

In the line

NSDictionary *_family = (NSDictionary *)_family;

you define a new variable _family in the current scope, which makes the outer variable _family invisible. Objective-C pointers are initialized to nil if you compile with ARC.

And the output is not contradictory, because you print

NSStringFromClass(children.class);

which is the class of children (without the underscore), which is an array. But _children (with underscore) is nil because _family is nil as explained above.

In fact you don’t need a type cast if you expect a dictionary. You could just do

NSDictionary *_family = [pedigree objectForKey:@"family"];
if ([_family isKindOfClass:[NSDictionary class]])
{
    NSArray *_children = [_family objectForKey:@"children"];

    if ([_children isKindOfClass:[NSArray class]]) {
        NSLog(@"Children is an NSArray");
    } else {
        NSLog(@"Children is not an NSArray");
    }
} else {
    NSLog(@"Family is not an NSDictionary");
}