Re: NSString** in scanners

FROM : Bill Bumgarner
DATE : Thu Jul 27 00:25:18 2006

On Jul 26, 2006, at 2:16 PM, D.K. Johnston wrote:
> I understand that if I want the scanned characters to be placed in:
>
>     NSString *string;
>
> I need to send "&string" to the method, which then presumably does 
> something like this:
>
>     *string = [internalWorkingString copy];
>
> But if the method got "string" instead of "&string", couldn't it 
> just do this:
>
>     string = [internalWorkingString copy];
>
> with the same result?


Much easier in code:


void foo(NSString *a, NSString **b) {
   NSLog(@"1. &a 0x%x", &a);
   NSLog(@"1. a 0x%x", a);
   NSLog(@"1. b 0x%x", b);
   NSLog(@"1. *b 0x%x", *b);
   
   a = [@"foo" copy];
   *b = [@"bar" copy];
   
   NSLog(@"2. a 0x%x", a);
   NSLog(@"2. b 0x%x", b);
   NSLog(@"2. *b 0x%x", *b);
}

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
   NSString *a = nil;
   NSString *b = nil;
   
   NSLog(@"0. &a 0x%x", &a);
   NSLog(@"0. a 0x%x", a);
   NSLog(@"0. b 0x%x", b);    
   NSLog(@"0. &b 0x%x", &b);    
   
   foo(a, &b);
   
   NSLog(@"3. a 0x%x", a);
   NSLog(@"3. b 0x%x", b);    
   NSLog(@"3. &b 0x%x", &b);    
   
    [pool release];
    return 0;
}

Which outputs:

2006-07-26 15:22:45.057 foobar[676] 0. &a 0xbffffa18
2006-07-26 15:22:45.057 foobar[676] 0. a 0x0
2006-07-26 15:22:45.057 foobar[676] 0. b 0x0
2006-07-26 15:22:45.057 foobar[676] 0. &b 0xbffffa14
2006-07-26 15:22:45.057 foobar[676] 1. &a 0xbffffa00
2006-07-26 15:22:45.057 foobar[676] 1. a 0x0
2006-07-26 15:22:45.057 foobar[676] 1. b 0xbffffa14
2006-07-26 15:22:45.057 foobar[676] 1. *b 0x0
2006-07-26 15:22:45.058 foobar[676] 2. a 0x2404c
2006-07-26 15:22:45.058 foobar[676] 2. b 0xbffffa14
2006-07-26 15:22:45.058 foobar[676] 2. *b 0x2405c
2006-07-26 15:22:45.058 foobar[676] 3. a 0x0
2006-07-26 15:22:45.058 foobar[676] 3. b 0x2405c
2006-07-26 15:22:45.058 foobar[676] 3. &b 0xbffffa14

Note that the address of a is different in foo() than in main(). 
Variables passed to functions are copied -- the bytes are copied, not 
it invokes -copy to copy the contents of the variable -- as they are 
passed to a function.  'b' works because you are a passing a pointer 
to a pointer to a string... the pointer to the pointer is copied and, 
thus, foo() can then set the pointer's value even though it is 
actually a copy of the pointer to the pointer that was passed in.

Perfectly clear, right?  :-)