Re: Use of Mac OS X 10.5 / Leopards Garbage Collection Considered Harmful

FROM : Clark Cox
DATE : Thu Feb 07 03:52:03 2008

On Feb 6, 2008 5:06 PM, John Engelhart <john.<email_removed>> wrote:
>
> Actually, I've thought of another example which addresses the use of
> (or lack of) __strong unambiguously and still demonstrates the problem:


No, you're just rehashing and reformulating the same argument over and
over again. You're not listening.


>
> #import <Foundation/Foundation.h>
>
> @interface GCTest : NSObject {
>    const char *title;
> };
>
> - (void)setTitle:(const char *)newTitle;
> - (const char *)title;
>
> @end
>
> @implementation GCTest
>
> - (void)setTitle:(const char *)newTitle
> {
>    printf("Setting title.  Old title: %p, new title %p = '%s'\n",
> title, newTitle, newTitle);
>    title = newTitle;


This is a bad thing to do in pre-GC Cocoa, it is still a bad thing to
do in post-GC Cocoa. Don't do it. If you want to store the result of
calling UTF8String, copy it. Period.

> }
>
> - (const char *)title
> {
>    return title;
> }
>
> @end
>
> int main(int argc, char *argv[]) {
>    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
>    GCTest *gcConstTitle = NULL, *gcUTF8Title = NULL;
>    void *ptr;
>
>    gcConstTitle = [[GCTest alloc] init];
>    gcUTF8Title = [[GCTest alloc] init];
>
>    [gcConstTitle setTitle:"Hello, world!"];
>    [gcUTF8Title setTitle:[[NSString stringWithUTF8String:"Hello, world \xC2\xA1"] UTF8String]];


At this point, the temporary NSString object has exactly zero roots.
Why are surprised that it goes away?

>    NSLog(@"Test: %@", @"hello");
>    [[NSGarbageCollector defaultCollector] collectExhaustively];
>    NSLog(@"GC test");
>
>    printf("gcConstTitle title: %p = '%s'\n", [gcConstTitle title], [gcConstTitle title]);
>    printf("gcUTF8Title  title: %p = '%s'\n", [gcUTF8Title title], [gcUTF8Title title]);
>
>    [gcConstTitle setTitle:NULL];  // Must clear the pointer before popping pool.
>    [gcUTF8Title setTitle:NULL];
>
>    [pool release];
>    return(0);
> }



> The above example is now perfectly legal by everyones definition of
> how things were under retain/release, and I correctly clear the
> pointer before it goes out of scope, and demonstrates that the GC
> system can, and does, reclaim live data out from under you.


No it is not. Holding on to the result of UTF8String for longer than
the lifetime of the NSString from which it came is not legal. How hard
is that to understand?


--
Clark S. Cox III
<email_removed>