EXC_BAD_ACCESS when using object_setIvar

Question

I am attempting to add and set Ivars on a runtime class that I've allocated using the code below. I do not have any experience with the objective-c runtime functions that's why I am trying to learn.

    if ([object isKindOfClass:[NSDictionary class]])
    {
        const char *className = [aName cStringUsingEncoding:NSASCIIStringEncoding];

        // Allocate the class using the class name, NSObject metaclass, and a size of 0
        Class objectClass = objc_allocateClassPair([NSObject class], className, 0);

        // Get all of the keys in the dictionary to use as Ivars
        NSDictionary *dictionaryObject = (NSDictionary *)object;
        NSArray *dictionaryObjectKeys = [dictionaryObject allKeys];

        for (NSString *key in dictionaryObjectKeys)
        {
            // Convert the NSString to a C string
            const char *iVarName = [key cStringUsingEncoding:NSASCIIStringEncoding];

            // Add the Ivar to the class created above using the key as the name
            if (class_addIvar(objectClass, iVarName, sizeof(NSString*), log2(sizeof(NSString*)), @encode(NSString*)))
            {
                // Get the newly create Ivar from the class created above using the key as the name
                Ivar ivar = class_getInstanceVariable(objectClass, iVarName);

                // Set the newly created Ivar to the value of the key
                id value = dictionaryObject[key];
                object_setIvar(objectClass, ivar, [value copy]);
            }
        }

        objc_registerClassPair(objectClass);
    }

Every time I run the above code I get a EXC_BAD_ACCESS (code=2, address = 0x10) error on the line object_setIvar(objectClass, ivar, [value copy]);. I don't understand why I am getting this error. I check if the Ivar was successfully added to the class before I attempt to set the Ivar's value, but apparently the ivar is nil. I've tried to NSLog the ivar, but I get the same error.

I tried searching on Google for a solution, but I can't find much information on the objective-c runtime functions.

I am using ARC and running the app on the iOS Simulator.

Answer 1

You cannot set values for the instance variables on the class. After creating and registering the class you can create an instance of the class:

id myInstance = [[objectClass alloc] init];

and then set values for the instance variables on this object:

for (NSString *key in dictionaryObjectKeys)
{
    const char *iVarName = [key cStringUsingEncoding:NSASCIIStringEncoding];

    id value = dictionaryObject[key];
    Ivar ivar = class_getInstanceVariable(objectClass, iVarName);

    object_setIvar(myInstance, ivar, [value copy]);
}