Fixing ARC error when using Objective-C object in struct

Question

I am having an issue with my Xcode project.

I have these lines:

typedef struct
{
    NSString *escapeSequence;
    unichar uchar;
}

and I am getting this error:

ARC forbids Objective-C objects in structs or unions.

How can I fix it?

I cannot seem to find how this violates ARC but I would love to learn.

possible duplicate of [Alternative to Objective-C objects in structs (ARC)](http://stackoverflow.com/questions/10055404/alternative-to-objective-c-objects-in-structs-arc) — Kurt Revis, Feb 09 '13 at 04:37
Also: [ARC forbids Objective-C objects in structs or unions despite marking the file -fno-objc-arc](http://stackoverflow.com/questions/8093099/arc-forbids-objective-c-objects-in-structs-or-unions-despite-marking-the-file-f), [Objective-C classes in structs with ARC](http://stackoverflow.com/questions/10851870/objective-c-classes-in-structs-with-arc), [the ARC documentation](http://clang.llvm.org/docs/AutomaticReferenceCounting.html#ownership-qualified-fields-of-structs-and-unions). — Kurt Revis, Feb 09 '13 at 04:39
brother, did u use the above code for epub reader. If so pls guide me to convert it to arc. It would be extremely helpful for me. — Arunavh Krishnan, May 20 '14 at 16:09

score 34 · Answer 1 · edited Feb 03 '15 at 10:43

34

Change it to:

typedef struct
{
    __unsafe_unretained NSString *escapeSequence;
    unichar uchar;
}MyStruct;

But, I recommend following Apple rules from this documentation.

ARC Enforces New Rules

You cannot use object pointers in C structures.
Rather than using a struct, you can create an Objective-C class to manage the data instead.

edited Feb 03 '15 at 10:43

Pang

9,564
146
81
122

answered Feb 09 '13 at 04:37

bitmapdata.com

9,572
5
35
43

1

This response is outdated. As of WWDC '18, you are now allowed to specify ARC object pointers in C structs. – Daniel J Mar 10 '19 at 02:25
It appears that at least __strong and __weak are now supported in structs by clang (2019). – Carl Lindberg Aug 16 '19 at 20:09

score 8 · Answer 2 · answered Mar 04 '15 at 01:50

The safest way is to use __unsafe_unretained or directly CFTypeRef and then use the __bridge, __bridge_retained and __bridge_transfer.

e.g

typedef struct Foo {
    CFTypeRef p;
} Foo;

int my_allocating_func(Foo *f)
{
    f->p = (__bridge_retained CFTypeRef)[[MyObjC alloc] init];
    ...
}

int my_destructor_func(Foo *f)
{
    MyObjC *o = (__bridge_transfer MyObjC *)f->p;
    ...
    o = nil; // Implicitly freed
    ...
}

score 1 · Answer 3 · edited May 31 '17 at 17:52

When we are defining C structure in Objective C with ARC enable, we get the error "ARC forbids Objective-C objects in struct". In that case, we need to use keyword __unsafe_unretained.

Example

struct Books{

    NSString *title;
    NSString *author;
    NSString *subject;
    int book_id;
};

Correct way to use in ARC enable projects:

struct Books{

    __unsafe_unretained NSString *title;
   __unsafe_unretained NSString *author;
   __unsafe_unretained NSString *subject;
   int book_id;
};

foldinglettuce · Answer 4 · 2014-05-23T20:21:05.123

0

I just integrated the same code into my project from the Google Toolbox for Mac

GTMNSString-HTML.m

Their suggestion for ARC Compatibility of adding the -fno-objc-arc flag to each file worked for me.

edited May 23 '14 at 20:21

answered Jul 30 '13 at 23:07

foldinglettuce

522
10
28

brother, did u use the above code for epub reader. If so pls guide me to convert it to arc. It would be extremely helpful for me.... – Arunavh Krishnan May 20 '14 at 16:10

Fixing ARC error when using Objective-C object in struct

4 Answers4

Linked