How to avoid temp objects when returning NSString under ARC - objective-c

Question

I've got a class with two properties:
#interface Contact : NSObject {
NSString *lastname;
NSString *lastNameUpper;
}
I've declared lastname as a property (and synthesize it in the .m-file):
#property (nonatomic, retain) NSString *lastname;
However, I want to write my own method to access the lastNameUpper, so I declared a method:
- (NSString *) lastNameUpper;
and implemented it like this:
- (NSString *) lastNameUpper {
if (!lastNameUpper) {
lastNameUpper = [lastname uppercaseString];
}
return lastNameUpper;
}
This works all right, but as this is called quite often, a lot of temporary objects are called. Interestingly, the Instruments show a lot of "Malloc (4k)", and the number increase each time lastNameUpper is accessed. I can also see that the memory is allocated in objc_retailAutoreleaseReturnValue.
As this was working fine before I converted my project to ARC, I'm assuming that I have to make some ARC specific additions to the method signature, but I can't seem to be able to make it work.
Any suggestions?

Answer 1

0: you should copy your NSString properties:
#property (nonatomic, copy) NSString * lastname;
I'm guessing that returning the string is implemented by copying it.
nope. copy of an immutable string is a retain operation. just run it in the profiler to see how much this costs in time and memory. also, there's no implicit copy in this case.
Update
I tested this on Lion-64. uppercaseString may return a mutable string.
To be safe, you may consider assigning a copy of the result of uppercaseString: lastNameUpper = [[lastname uppercaseString] copy];. that may result in more or less allocations, depending on how you used the string in your implementation. if your properties copy, then a copy will be made each time you assign it. the easy generalization is to assign a copy, and the rest usually takes care of itself.
Test Program
// ARC enabled
#import <Foundation/Foundation.h>
#interface Contact : NSObject
{
NSString * lastname;
NSString * lastNameUpper;
}
#property (nonatomic, copy) NSString *lastname;
#end
#implementation Contact
#synthesize lastname;
- (NSString *) lastNameUpper {
if (!lastNameUpper) {
lastNameUpper = [lastname uppercaseString];
}
return lastNameUpper;
}
#end
int main(int argc, const char * argv[]) {
#autoreleasepool {
int n = 0;
while (n++ < 100000) {
Contact * c = [Contact new];
c.lastname = #"skjdhskjdhaksjhadi";
NSString * lastNameUpper = c.lastNameUpper;
}
}
return 0;
}

Answer 2

Override the - (void)setLastname:(NSString*)aLastname method (created automatically by #synthesize lastname, and set lastNameUpper as in the existing method.
Now create a lastNameUpper property (and synthesize it):
#property (nonatomic, readonly) NSString *lastNameUpper;
Since this will return the pointer of the lastNameUpper instance variable, no copies should be made whenever this is accessed.