If I want to make a new instance of an object in a function whose pointer is passed by reference in it.

Question

- (void)createAString:(NSString **)str
{
   *str = [NSString stringWithString:@"Hi all!"];
   [*str autorelease]; // ???? is this right ?
}

How should I use release or autorelease ? I don't want to release outside of the function of course :)

...
NSString *createStr;
[self createAString:&createStr];
NSLog(@"%@", createStr);

Answer 1

Instead of using a double pointer, would it not be more elegant to use an NSMutableString instead?

- (void)createAString:(NSMutableString *)str
{
    [str setString:@"Hi all!"];
}

....

NSMutableString *createStr = [[NSMutableString alloc] init];
[self createAString: createStr];
NSLog(@"%@", createStr);
[createStr release];

---

Or, even better, just have the createAString method return an NSString.

- (NSString *)createAString
{
    return @"Hi all!"; // this is autoreleased automatically
}

I wouldn't want to presume that your needs are this simple, though. =)

Answer 2

You're correct that you'd generally want to return autoreleased (or the like) objects from out params when you use this form. Your assignment statement in the function that sets *str to a string:

*str = [NSString stringWithString:@"foo"];

is already doing the right thing, because that method returns an instance of NSString that the caller doesn't own. Just like you could return this string object from your function without any further memory management, you can set it as the outparam as you've done. Your second snippet showing the call site is fine.

---

This said, I'm worried about a few things in your code that you should be sure you understand:

1. The value of str inside the method is still a **, and sending that a message (as you've done for the speculative autorelease) is nonsense. Be sure you fully understand doubly indirected pointers before using them too liberally. :) If you need to send str a message after creating it, send it to *str, which is what contains the NSString *.

2. Setting an outparam like this when the function returns void is not idiomatic Cocoa. You would normally just return the NSString * directly. Outparams are rare in Cocoa. (Usually just NSErrors get this treatment from framework calls. Otherwise they conventionally use name like getString to differentiate them from normal get accessors which don't use the word "get".)

3. I hope -stringWithString was just an example. That method is almost never used in practice, since it's equivalent (in this case) to just using a @"string literal" (although that would muddy your example).