Re: Garbage collector vs variable lifetime
Re: Garbage collector vs variable lifetime
- Subject: Re: Garbage collector vs variable lifetime
- From: Ken Thomases <email@hidden>
- Date: Fri, 6 Jun 2008 23:16:28 -0500
On Jun 6, 2008, at 10:48 PM, Antonio Nunes wrote:
On 7 Jun 2008, at 01:42, Bill Bumgarner wrote:
The easiest way to do this is to simply to use data once after the
for() loop:
NSData* data = <get it from somewhere>;
const unsigned char* bytes = [data bytes];
NSUInteger count = [data length];
for (NSUInteger i = 0; i < count; i++)
something = bytes [i];
[data self];
Yup. I don't particularly like it either. Fortunately, it is an
uncommon case -- most of the objects in the AppKit/Foundation "just
work". This kind of a problem arises when something leverages the
C in Objective-C.
Quincey Morris wrote:
A little inner voice insists on asking, though, how we know some
future version of the compiler might not optimize '[data self]'
upwards before the loop, if it decides that nothing inside the loop
references anything non-local:
Why not explicitly turn off collection for the data pointer:
NSData* data = <get it from somewhere>;
[[NSGarbageCollector defaultCollector]
disableCollectorForPointer:data];
const unsigned char* bytes = [data bytes];
NSUInteger count = [data length];
for (NSUInteger i = 0; i < count; i++)
something = bytes [i];
[[NSGarbageCollector defaultCollector]
enableCollectorForPointer:data];
It's slightly longer, but straightforward and clear about it's
purpose, and should be future proof.
And... we're back to retain/release.
The issue is, how can one know when this technique is necessary? The
supposed rules of GC are, if it's in a stack variable, it's safe. The
compiler here is doing something behind the programmer's back, which
the programmer can't really predict, that's changing the rules.
Quoting the Garbage Collection Programming Guide: "The initial root
set of objects is comprised of global variables, stack variables, and
objects with external references. These objects are never considered
as garbage"
For some values of "never".
Objective-C 2.0's garbage collection system is incompatible with any
optimization which changes which variables are on the stack. More
accurately, it's an error if compiler optimizations change what are
considered root objects.
Regards,
Ken
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