A std::tr1::shared_ptr for Objective C++ on iPhone ???

Question

Hi, I am mostly a C++ developer, recently I am writing iPhone applications.

The memory management on iPhone is OK to me, due to resource limitation, it's encouraged to use reference counters rather than deep copy.

One annoying thing is I have to manage the reference counters by myself: alloc means counter = 1; retain means counter++, release means counter--

I wish to write a shared_ptr like class for Cocoa Touch, so I rarely have to manually manipulate the reference counters by myself.

I doubt if there's any existing code for that, and I'd like to hear some advices, today is the 5th day since I started to learn objective c

Thanks.

Answer 1

Resource management in Cocoa can be tricky: some API calls automatically retain a reference and some don't, some return an autoreleased object, some a retained object. By shielding this in a shared_ptr class, you'll more likely to make mistakes. My advice is to first take the "normal" Cocoa route until you're fairly experienced.

Answer 2

You forgot case 4

[4] you need to pass a pointer to an object out of a method as the return value.

This is where you need -autorelease.

I suggest you read the memory management rules and write some real code before you attempt this little project so that you can get a feel of how memory management is supposed to work.

Answer 3

Have you looked into [object autorelease]? Perhaps that would make things a bit easier.

Answer 4

As long as you learn the memory management rules first, there is no real problem with shared_ptr - it can help you in C++ contexts but doesn't let the ownership questions magically disappear.
shared_ptr supports a custom deallocator so the following:

@interface A : NSObject
- (void)f;
@end

@implementation A
- (void)dealloc { NSLog(@"bye"); [super dealloc]; }
- (void)f { NSLog(@"moo"); }
@end

void my_dealloc(id p) {
    [p release];
}

// ...
{
    shared_ptr<A> p([[A alloc] init], my_dealloc);
    [p.get() f];
}

... outputs:

moo
bye

... as expected.

If you want you can hide the deallocator from the user using a helper function, e.g.:

template<class T> shared_ptr<T> make_objc_ptr(T* t) {
    return shared_ptr<T>(t, my_dealloc);
}

shared_ptr<A> p = make_objc_ptr<A>([[A alloc] init]);