Accessing a class ivar from a C function

Question

I have a C function in a class (yes, has to be in C) and I need to access an ivar from that class. The ivar in question is a NSMutableDictionary.

From what I understand, C functions don't have direct access to ivars of a class and a reference must be passed to them. So, I have added this before the implementation

static NSMutableDictionary *myIvarRef;

and did this on the init of that class.

- (id)init
{
    self = [super init];
    if (self != nil)
    {
        myIvar = [[NSMutableDictionary alloc] init];
        myIvarRef = myIvar;
    }
    return self;
}

later, inside that function, when I try to use myIvarRef it is nil.

What am I missing?

thanks

---

The C function is complex to put here, but the line that I am trying to use is like this

MyObject obj = myIvarRef[myKey];

I put a breakpoint at this line and when it stops I type po myIvarRef on console and it gives me nil.

Answer 1

Your code has the big problem that the static myIvarRef variable is overwritten in each init call, and therefore always points to the ivar of the last created object.

A better solution would be to pass the object pointer as an additional argument to the C function. Something like this:

void myFunction(void *uData, /* other parameters */ )
{
    MyClass *obj = (__bridge MyClass *)(uData);
    // Now you can access all properties, ivars etc of obj.   
    // ...
}

- (void)myMethod
{
    // Call C function from Objective-C method:
    myFunction((__bridge void *)(self), /* other arguments */ );
}