Ternary default on boxed boolean with unset key for NSDictionary - objective-c

Question

So I've tested this, but just wanting to make sure it wasn't some random undefined behavior. I want to use the shorthand ternary on a dictionary but I want it to return a default value when it is not set. Here is the test I wrote:
NSMutableDictionary *test = [NSMutableDictionary new];
test[#"test_1"] = #NO;
test[#"test_2"] = #YES;
BOOL test1 = [test[#"test_1"] boolValue] ?: NO;
BOOL test2 = [test[#"test_2"] boolValue] ?: NO;
BOOL test3 = [test[#"test_3"] boolValue] ?: NO;
LogDebug(#"test1 = %#", (test1 ? #"YES" : #"NO"));
LogDebug(#"test2 = %#", (test2 ? #"YES" : #"NO"));
LogDebug(#"test3 = %#", (test3 ? #"YES" : #"NO"));
Now I got the correct value for test3, but I'm wondering whether thats just a random fluke of undefined behavior. I'm wondering this because when I checked for that value in the debugger by typing this in the console:
po [test[#"test_3"] boolValue]
I got:
error: no known method '-boolValue'; cast the message send to the method's return type
error: 1 errors parsing expression
Are the results of test3 reliable?

Answer 1

NSDictionary (and its subclass, NSMutableDictionary) is documented as returning nil for a key with no value.
In Objective-C, sending a message to nil is documented to return NO when you expect a BOOL result.
Therefore, if you want the default value to be NO, this suffices:
BOOL test3 = [test[#"test_3"] boolValue];
If you only intend to store NSString keys and NSNumber values (which #YES and #NO are) in the dictionary, you can declare it like this:
NSMutableDictionary<NSString *, NSNumber *> *test = [NSMutableDictionary new];
And then you can use a property accessor if you prefer:
BOOL test3 = test[#"test_3"].boolValue;
If you want a different default value (and the only other possibility for BOOL values is YES), then #trojanfoe's suggestion of creating a method is what I'd recommend, but I'd probably make a more general category on NSDictionary like this:
// NSDictionary+kailoon.h
#interface NSDictionary<KeyType, ObjectType> (kailoon)
- (ObjectType)kailoon_objectForKey:(KeyType)key default:(ObjectType)defaultObject;
#end
// NSDictionary+kailoon.m
#implementation NSDictionary (kailoon)
- (id)kailoon_objectForKey:(id)key default:(id)defaultObject {
id value = self[key];
return value ? value : defaultObject;
}
#end
Then I'd use it like this:
BOOL test3 = [test kailoon_objectForKey:#"test_3" default:#YES].boolValue;
As for your debugger problem, it's incorrect to use po to print a BOOL, because po means “print object” and a BOOL is not an object. (It is a primitive.) So you should just use p. But that's not really the problem here. To fix the “no known method” problem, you use a cast, like this:
(lldb) p (BOOL)[test[#"test_3"] boolValue]
(BOOL) $1 = NO

Answer 2

The output you are getting from your code is correct and defined.
First off, your code can be simplified to:
BOOL test1 = [test[#"test_1"] boolValue];
BOOL test2 = [test[#"test_2"] boolValue];
BOOL test3 = [test[#"test_3"] boolValue];
You will get NO for any key that doesn't exist. If test[#"some_nonexistent_key"] returns nil, calling boolValue on nil results in NO.
The debugger is just getting confused because it doesn't know what to do with the call to boolValue on nil since there are more than one possible boolValue methods (NSNumber and NSString).

Answer 3

That code is reliable, but only for booleans and only when the default value is NO, as non-existence of the value in the dictionary will give NO and hence the default value will be used.
However it's somewhat clumsy as:
[test[#"test_3"] boolValue] ?: NO;
is basing the result of the ternary expression on whether the boolean stored in the NSNumber is YES or NO, when what you want to know is is the value set in the dictionary?.
This is better:
BOOL test1 = test[#"test_1"] != nil ? [test[#"test_1"] boolValue] : NO;
and will use the default value only if the dictionary does not contain the specified key. This pattern can also be extended to other types and can also be used if the default value is YES.
Wrapped into a method (recommended), it would be the following (this code also checks the value type):
- (BOOL)boolForKey:(NSString *)key defaultValue:(BOOL)defaultValue
{
id value = self.dictionary[key];
return [value isKindOfClass:[NSNumber class]] ? [value boolValue] : defaultValue;
}