Trying to discern a mutable dictionary from an immutable one.

mutant.m

#import <Foundation/Foundation.h>

// clang -g -framework Foundation -o mutant mutant.m

int main (void) {
    NSMutableDictionary *mutable = [[NSMutableDictionary alloc] init];
    NSDictionary *immutable = [[NSDictionary alloc] init];

    NSLog (@"mutable responds to set object for key: %d",
           [mutable respondsToSelector: @selector(setObject:forKey:)]);
    NSLog (@"immutable responds to set object for key: %d",
           [immutable respondsToSelector: @selector(setObject:forKey:)]);

    NSLog (@"mutable is kind of class NSDictionary: %d",
           [mutable isKindOfClass: [NSDictionary class]]);
    NSLog (@"immutable is kind of class NSDictionary: %d",
           [immutable isKindOfClass: [NSDictionary class]]);

    NSLog (@"mutable is kind of class NSMutableDictionary: %d",
           [mutable isKindOfClass: [NSMutableDictionary class]]);
    NSLog (@"immutable is kind of class NSMutableDictionary: %d",
           [immutable isKindOfClass: [NSMutableDictionary class]]);

    NSLog (@"mutable is member of class NSDictionary: %d",
           [mutable isMemberOfClass: [NSDictionary class]]);
    NSLog (@"immutable is member of class NSDictionary: %d",
           [immutable isMemberOfClass: [NSDictionary class]]);

    NSLog (@"mutable is member of class NSMutableDictionary: %d",
           [mutable isMemberOfClass: [NSMutableDictionary class]]);
    NSLog (@"immutable is member of class NSMutableDictionary: %d",
           [immutable isMemberOfClass: [NSMutableDictionary class]]);

    NSLog (@"mutable is of class %@", [mutable class]);
    NSLog (@"immutable is of class %@", [immutable class]);


    // Reach into the guts.  Please don't do this in production code.
    typedef struct RuntimeBaseHackBlorf {
        uintptr_t _cfisa;
        uint8_t _cfinfo[4];
    } RuntimeBaseHackBlorf;
    
#define BIT_SIX (1 << 6)
#define BIT_SIX_SET(x)  (((RuntimeBaseHackBlorf *)(x))->_cfinfo[0] & BIT_SIX)

    NSLog (@"mutable has bit 6 set: %d", BIT_SIX_SET(mutable));
    NSLog (@"immutable has bit 6 set: %d", BIT_SIX_SET(immutable));
    
    // Check another Basic Hash
    NSMutableSet *mutableSet = [NSMutableSet set];
    NSSet *immutableSet = [NSSet set];

    NSLog (@"mutable set has bit 6 set: %d", BIT_SIX_SET(mutableSet));
    NSLog (@"immutable set has bit 6 set: %d", BIT_SIX_SET(immutableSet));

    return 0;

} // main