What is NSFrozenDictionaryM?

Question

I came across NSFrozenDictionary while debugging an app.

Shared index property declared as NSDictionary * sharedIndex = ...

What is it? How is it different from NSMutableDictionary?

Answer 1

It is an NSMutableDictionary marked as immutable.

One case to get __NSFrozenDictionaryM:

1. Have an array of mutable dictionaries:

   NSArray *array = @[{NSMutableDictionary}, {NSMutableDictionary}, {NSMutableDictionary}]

2. Making a two-level-deep copy of it by:

   NSArray *res = [[NSArray alloc] initWithArray:array copyItems:YES]

The resulting res array contains immutable copies of NSMutableDictionaries in array, which are of type __NSFrozenDictionaryM. I guess this is an optimisation to avoid really copying all dictionaries in the original array.