Question

I want to pass 2 variables:

UIImage * img
int i

into another method that only takes a (void *)

I tried making a C struct containing both img and i

struct MyStruct {
    UIImage *img;
    int i;
}

but xcode gives me an error saying “ARC forbids Objective-C objects in structs or unions”

The next thing I tried is to write an objective-c class MyStruct2 containing img and i, alloc-initing an instance of it and typecasting it as (__bridge void*) before passing it to the method. Seems little involved for my use case. Seems like there should be a better way.

What’s the simplest way to achieve this?

Thank you.

Edit based on comments: I have to use void * as it is required by the UIView API. I created a selector as mentioned by UIVIew API

+ (void)setAnimationDidStopSelector:(SEL)selector

Please see documentation for setAnimationDidStopSelector at http://developer.apple.com/library/ios/#documentation/uikit/reference/UIView_Class/UIView/UIView.html . It says … The selector should be of the form:

    - (void)animationDidStop:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context 

I want to pass both img and i into the (void *)context argument.

Answer 1

ARC cannot follow ObjC objects in plain old C structs. It tracks pointers on the stack, or in ObjC and C++ objects, which are copied using methods, explicit or implicit, but under the compiler control anyway.

So… use a class. C++ if you like it, or :

@interface MyNotSoPlainStruct

@property ( nonatomic, strong ) UImage* image;
@property ( nonatomic ) int i;

@end

@implementation MyNotSoPlainStruct
@end

MyNotSoPlainStruct* foo = [MyNotSoPlainStruct new];
foo.image = …
foo.i = …

and there you go.

The solution needs more lines, that’s true, but you have a much better control on how things are done, including the image life cycle. You can use a strong pointer, or a weak one if it suits your needs better.

Of course, you can also use either a dictionary or an array. Literals make that especially easy:

NSDictionary* arguments = @{ @"image" : <image>, @"i" : @( <int> ) };
arguments[@"i"].intValue returns the int you need.

Pretty easy.

edit

Forgot the transfer part, oops :). So then:

You must use the following annotations:

void* argumentsPointer = (__bridge_retained void*) arguments;

arguments is retained, and that reference is no longer managed by ARC.

When you receiver your pointer back, you must convert it:

NSDictionary* arguments = (__bridge_transfer NSDictionary*) argumentsPointer;

Using __bridge only may result in a dangling pointer if the last known reference disappear. If you want that void* to retain the dictionary, you must use __bridge_retained first, then __bridge_transfer.