On May 12, 2012, at 12:17 PM, Ken Thomases wrote:
> That's not necessarily so. And/or requesting the mutableBytes may do the
> equivalent of retain+autorelease on the NSMutableData.
>
> Consider an inexact analog. The -[NSString UTF8String] method seems to
> create an autoreleased NSData (or similar object) to hold the UTF-8-encoded C
> string that it returns. It's not returning the NSData object, obviously, it's
> just using the autoreleased lifetime to manage the lifetime of the C string.
>
> Anyway, NSMutableData *could* be doing something similar. I don't know if it
> is or isn't. I believe that Andreas is saying that his testing shows that it
> is. If it is, Apple may have done this specifically to avoid a problem with
> internal pointers in ARC.
It looks like that’s indeed what it’s doing. I put a category on NSData to
swizzle out its dealloc to something that would log that it was getting
dealloced, and then ran a few test cases. Here’s where the data object got
dealloced each time:
int main(int argc, const char * argv[]) {
@autoreleasepool {
const char *bytes = "abcd";
{
NSMutableData *data = [[NSMutableData alloc] initWithBytes:bytes
length:4];
NSLog(@"leaving block");
} // data gets dealloced here
NSLog(@"left block");
}
return 0;
}
int main(int argc, const char * argv[]) {
@autoreleasepool {
const char *bytes = "abcd";
char *mutableBytes;
{
NSMutableData *data = [[NSMutableData alloc] initWithBytes:bytes
length:4];
mutableBytes = [data mutableBytes];
NSLog(@"leaving block");
}
NSLog(@"left block");
} // data gets dealloced here
return 0;
}
int main(int argc, const char * argv[]) {
@autoreleasepool {
const char *bytes = "abcd";
char *mutableBytes;
@autoreleasepool {
NSMutableData *data = [[NSMutableData alloc] initWithBytes:bytes
length:4];
mutableBytes = [data mutableBytes];
NSLog(@"leaving autoreleasepool block");
} // data gets dealloced here
NSLog(@"left autoreleasepool block");
}
return 0;
}
Charles
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