Question

I’m not sure what I’m doing wrong here. I’ve also tried setting s1..3 in foo by using:

s1 = [[NSString alloc] initWithString:[filepaths objectAtIndex:0]];

Context below:

void foo(NSString *s1, NSString *s2, NSString *s3){
    //assign long string to NSString *fps
    //...
    //break fps into smaller bits
    NSArray *filepaths = [fps componentsSeparatedByString:@"\n"];
    //the above worked! now let's assign them to the pointers
    s1 = [filepaths objectAtIndex:0];
    //repeat for s2 and s3
    NSLog(@"%@",s1); //it worked! we're done in this function
}

int main(int argc, const char * argv[]){
    NSString *s1 = nil; //s2 and s3 as well
    foo(s1,s2,s3); //this should work
    NSLog(@"%@",s1); //UH OH, this is null!
    return 0;
}

Answer 1

No.

You are passing in pointers to objects which can be mutated locally. You are not changing the original objects, as you might think from plain C.

If you want to use this method (which I would not recommend – it’s really odd to see in Cocoa except in the case of NSError), you would have something like:

void foo(NSString **s1, NSString **s2, NSString **s3) {
    *s1 = [filepaths objectAtIndex:0]; // etc.
}

You would then pass in &s1 as the argument.

This will, of course, clobber whatever was in s1, potentially cause memory leaks, thread unsafety, etc., unless you are really careful. Which is why I say you usually won’t do this.