stringWithFormat vs. initWithFormat on NSString

Question

I am wondering what differences such as disadvantages and/or advantages there are to declaring an NSString this way:

NSString *noInit = [NSString stringWithFormat:@"lolcatz %d", i];

as opposed to:

NSString *withInit = [[NSString alloc] initWithFormat:@"Hai %d", i];

What was the motivation of putting stringWithFormat instead of just having the initWithFormat way of initializing the string?

Answer 1

stringWithFormat: returns an autoreleased string; initWithFormat: returns a string that must be released by the caller. The former is a so-called "convenience" method that is useful for short-lived strings, so the caller doesn't have to remember to call release.

Answer 2

I actually came across this blog entry on memory optimizations just yesterday. In it, the author gives specific reasons why he chooses to use [[NSString alloc] initWithFormat:@"..."] instead of [NSString stringWithFormat:@"..."]. Specifically, iOS devices may not auto-release the memory pool as soon as you would prefer if you create an autorelease object.

The former version requires that you manually release it, in a construct such as this:

NSString *remainingStr = nil;
if (remaining > 1)
    remainingStr = [[NSString alloc] initWithFormat:@"You have %d left to go!", remaining];
else if (remaining == 1)
    remainingStr = [[NSString alloc] initWithString:@"You have 1 left to go!"];
else
    remainingStr = [[NSString alloc] initWithString:@"You have them all!"];

NSString *msg = [NSString stringWithFormat:@"Level complete! %@", remainingStr];

[remainingStr release];

[self displayMessage:msg];

Here, remainingStr was only needed temporarily, and so to avoid the autorelease (which may happen MUCH later in the program), I explicitly handle the memory as I need it.