id array member instance under ARC

Question

I want to write something like this:

@interface Foo{

    __strong id idArray[]; 

}
@end

But the compiler complains about it:

Field has incomplete type '__strong id []'.

How can I create an id array member instance under ARC? And how do I init that array? Using malloc? new[]?

I don't want to use NSArray because I'm converting a large library to ARC and that will cause a lot of work.

Why do you convert an already-working library to ARC? ARC is a per-file technology. — Yuji, Sep 05 '11 at 12:52
How big is this library, really? Is it worth the hassle of dealing with C arrays and boxing/unboxing to work with the rest of your program? Most existing users won't particularly mind a conversion process on first launch (heck, Mail does that); new users won't have anything to convert. And you shouldn't be storing a large array into RAM either… — FeifanZ, Sep 05 '11 at 13:06
Hi Yuji and Inspire48, Actually, the library is not an traditional library, it's https://github.com/booyah/protobuf-objc. The library will generate objective c source files based on *.proto files. And the generated *.m files will be included in my project directly, which is using ARC. I've tried using NSArray, but came up with tons of compile errors and cost me a whole day to fix that.So maybe the simplest way is to use id[]. — Jay Zhao, Sep 05 '11 at 14:18

Kazuki Sakamoto · Accepted Answer · 2011-09-06T04:45:18.497

11

If you want to allocate dynamically the array, use pointer type of id __strong.

@interface Foo
{
    id __strong *idArray;
}
@end

Allocate the array using calloc. id __strong must be intialized with zero.

idArray = (id __strong *)calloc(sizeof(id), entries);

When you are done, you must set nil to the entries of the array, and free.

for (int i = 0; i < entries; ++i)
    idArray[i] = nil;
free(idArray);

edited Sep 06 '11 at 04:45

answered Sep 05 '11 at 12:57

Kazuki Sakamoto

Thanks Kazuki Sakamoto, it works! But the params passing to calloc seems to be like this: – Jay Zhao Sep 05 '11 at 14:23
Oh, I fixed the 1st param for calloc. – Kazuki Sakamoto Sep 06 '11 at 04:45
Thanks for actually providing a useful and correct answer, rather than asking why. I just ran into this for new code, and was glad for the help. – Quinn Taylor Jul 31 '12 at 01:07
2

It's worth noting that zeroing the array (with calloc, bzero, memset, etc.) up front is critical, or ARC will try to release garbage non-zero values when you assign a value. (See http://stackoverflow.com/a/9119631/120292 for details.) Also, failing to nil out all values before freeing the array will result in a leak. – Quinn Taylor Jul 31 '12 at 06:08

Georg Fritzsche · Answer 2 · 2011-09-05T12:47:14.980

1

You have to specify an array size, e.g.:

__strong id idArray[20];

edited Sep 05 '11 at 12:47

answered Sep 05 '11 at 12:41

Georg Fritzsche

score 1 · Answer 3 · answered Sep 05 '11 at 12:59

1

Either you give the array a fixed size:

__strong id idArray[20];

or you use a pointer and malloc:

__strong id *idArray;

...

self.idArray = calloc(sizeof(id), num);

answered Sep 05 '11 at 12:59

Rudy Velthuis

3 Answers3