Out parameters in ARC Objective-C

Question

I'm using Objective-C, and I don't know how to create and call a method with out parameters when compiling the code with the ARC compiler.

This is the kind of thing I'm trying to accomplish in non-ARC Objective-C (this is probably wrong anyway).

//
//  Dummy.m
//  OutParamTest

#import "Dummy.h"

@implementation Dummy

- (void) foo {
    NSString* a = nil;
    [self barOutString:&a];
    NSLog(@"%@", a);
}

- (void) barOutString:(NSString **)myString {
    NSString* foo = [[NSString alloc] initWithString:@"hello"];
    *myString = foo;
}

@end

I've read the documentation here: https://clang.llvm.org/docs/AutomaticReferenceCounting.html

and here: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocProperties.html

...but am finding it difficult to get anything that compiles, never mind anything that is correct. Would anybody be able to rewrite the jist of the code above, in a way that is suitable for ARC Objective-C?

Scutate

What exactly is your problem? What compiler errors do you get? – Bibeau

I've updated the code sample above with your suggestions, and included the set of compiler errors. Thanks. – Scutate

See my updated answer. As the compiler says, you cannot use an indirect pointer like that, instead you must pass in &a directly. – Bibeau

Thanks for your help @MikeWeller, the code above compiles now - is it technically correct? – Scutate

You don't need the b variable, just pass in &a as the argument. You also don't need to use __autoreleasing on the a and b variables unless you need to access the initial value in barOutString: which isn't very common. – Bibeau

Yep, sorry, missed your comment. I was getting very muddled with the pointers there. Looks ok now then? Thanks. – Scutate

NSString* foo = [[[NSString alloc] initWithString:@"hello"] autorelease]; Gives me "ARC forbids explicit message send of 'autorelease'" This compiles though: NSString __autoreleasing* foo = [[NSString alloc] initWithString:@"hello"]; – Scutate

Arg. What was I thinking? Sorry, yes the code is correct now. Of course you don't need autorelease with ARC -_- – Bibeau

Answer 1

You need to use the __autoreleasing attribute on the out parameter:

- (void) barOutString:(NSString * __autoreleasing *)myString {
    NSString* foo = [[NSString alloc] initWithString:@"hello"];
    *myString = foo;
}

The prerelease documentation (which I'm not allowed to link to due to NDA) puts the __autoreleasing in the middle of the two '*'s, but it might just work as (__autoreleasing NSString **)

You also cannot use an indirect double pointer (b) as in your original code. You must use this form:

- (void) foo {
    NSString* a = nil;
    [self barOutString:&a];
    NSLog(@"%@", a);
}

You are also calling dealloc directly on an object which is completely wrong. I suggest you read the memory management guidelines.

Bibeau

There's a small error in the first method. *myString = &foo; should be changed to *myString = foo;. – Syncopated

Thanks, fixed. Not sure how that got in there. – Bibeau