On Apr 3, 2009, at 19:32, Dave Keck wrote:
As for read-only address space, does the const declaration not take
care of
that? (Serious question.)
I just tested whether it's possible to write at the address of an
NSString declared with either:
NSString *const HelloString = @"ABC123";
#define GoodbyeString @"XYZ345"
(If you're interested, you can download my test project here:
http://www.docdave.com/halla.zip)
In both cases, I was able to write to the address of the string
without crashing. I found this surprising, because I assumed at least
HelloString would be stored in the read-only _TEXT segment...
Assuming I've got my const's straight, "NSString *const" makes the
pointer const, but not the thing pointed to -- "const NSString *const"
would make the pointer *and* the value const, but I doubt the compiler
actually has the concept of const-ness of objects. (And NSString's
immutability is a run-time thing.)
The object address you used in your test was the NSString instance's
address, not the address of the actual characters. What you actually
clobbered, if I read it right, is the instance's isa pointer. It's
still possible that the C-string is somewhere readonly, and is
converted to something at run-time (e.g. UTF-8) or it's possible that
the NSString implementation refers to the C-string internally without
conversion.
Anyway, that's all about implementation, and there may be multiple
implementations, depending on the string's content, or the way it's
used. All I was trying to say earlier was that I'm happy enough to
take whatever the compiler chooses to do.
If in fact there's any reason to follow Apple's quoted recommendation
against the #define approach to string literals, I'd love to hear it.
Type safety isn't a consideration, I think, because @"..." is
unambiguous as to type. (Well, maybe ambiguous as to const-ness, I
don't know.)
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