Question

 NSString *s1 = @"string1";
 // NSString *s2 = [[NSString alloc]stringWithFormat:@"string2"];
 NSString *s2 = [[NSString alloc] initWithFormat:@"string2"];

I know this is pretty basic concept but I am not 100% clear with this.

First one doesn’t allocate a memory address and send one allocates a memory address…
Also second one increases the reference count of the variable but not the first one…
Even though I understand the concept but don’t know the situation when I should use the first one or the second one..

When should I use the first one ? also when should use for the second one?

Thanks in advance..

Answer 1

You should never use the second one – +stringWithFormat: is a class method. Including your first example, you have basically 3 choices:

NSString *s1 = @"string1";
NSString *s2 = [[NSString alloc] initWithString:@"string2"];
NSString *s3 = [NSString stringWithFormat:@"string3"];

s1 in this case is a pointer to a constant string. s2 and s3 both point to new strings that you’ve created, but s2 has been retained for you, and s3 has been autoreleased. If you just need a temporary object, the autoreleased object s3 or the constant object s1 are good choices. If you need to keep the string around, you should use s2 (actually now that I think about it s1 will work in this case too – it’s not really idiomatic, though).

You can make s1 or s3 equivalent to s2 by sending them a retain message. You could also turn s2 into an equivalent object by sending it an autorelease message.