With ARC, what's better: alloc or autorelease initializers?

Question

Is it better (faster & more efficient) to use alloc or autorelease initializers. E.g.:

- (NSString *)hello:(NSString *)name {
          


        

Answer 1

I disagree with the other answers, the autorelease version (your 2nd example) is not necessarily better.

The autorelease version behaves just as is it did before ARC. It allocates and inits and then autoreleases, which means the pointer to the object needs to be stored to be autoreleased later the next time the autorelease pool is drained. This uses slightly more memory as the pointer to that object needs to be kept around until it is processed. The object also sticks around longer than if it was immediately released. This can be an issue if you are calling this many times in a loop so the autorelease pool would not have a chance to be drained. This could cause you to run out of memory.

The first example behaves differently than it did before ARC. With ARC, the compiler will now insert a "release" for you (NOT an autorelease like the 2nd example). It does this at the end of the block where the memory is allocated. Usually this is at the end of the function where it is called. In your example, from viewing the assembly, it seems like the object may in fact be autoreleased. This might be due to the fact the compiler doesn't know where the function returns to and thus where the end of the block is. In the majority of the cases where a release is added by the compiler at the end of a block, the alloc/init method will result in better performance, at least in terms of memory usage, just as it did before ARC.