arguments by reference in objective-c?

Question

Hi, I'm trying to pass a nsstring by reference but it doesn't work.

this is the function:

+(void)fileName:(NSString *) file
{
    file = @"folder_b";
}

and this is the call:

NSString *file;

[function fileName:file];

nslog(@"%@",file);    // and there is nothing in the string....

What I must do to pass my string byref.

Answer 1

I suspect this is because NSString is immutable. Have you tried NSMutableString?

Answer 2

I believe you're looking for:

+ (void)fileName:(NSString **)file
{
  *file = @"folder_b";
}

What's really done here is we're working with a pointer to a pointer to an object. Check C (yup, just plain C) guides for "pointer dereference" for further info.

(...But as has been pointed out repeatedly, you're asking the wrong question.)

Answer 3

If you have want to return a value, then return a value. Pass by reference in Cocoa/iOS is largely limited to NSError**.

Given:

+(void)fileName:(NSString *) file

Then do:

+(NSString *) fileName;

And be done with it.

If you need to return more than one value at a time, that begs for a structure or, more often, a class.

In Objective-C, pass by reference smells like you are doing it wrong.