Question

I was having an issue with my UPDATE statement as I was telling here: Update issue with sqliteManager

I found out that initWithFormat WORKS

NSString *sqlStr = [[NSString alloc] initWithFormat:@"UPDATE User SET Name = :Name WHERE Id = :Id"];

BUT not stringWithFormat:

 NSString* sqlStr = [NSString stringWithFormat:@"UPDATE User SET Name = :Name WHERE Id = :Id"];

Why is this as such? I would like to understand the logic/reasoning behind..

Answer 1

I have just found something interesting from this thread: How to refresh TableView Cell data during an NSTimer loop

This, I believe, is the reasoning behind..

I quote what “petergb” said:

[NSString stringWithFormat:…] returns an autoreleased object. Autoreleased objects get released after control returns from the program’s code to the apple-supplied run-loop code. They are more or less a convenience so we don’t have to release all the little objects that we use once or twice here and there. (For example, imagine how tedious it would be if you had to release every string you created with the @”” syntax…)

We can tell stringWithFormat: returns an autoreleased object because, by convention, methods who’s names don’t start with alloc or copy always return auto-released objects. Methods like this are said to “vend” an object. We can use these objects in the immediate future, but we don’t “own” it (i.e. we can’t count on it being there after we return control to the system.) If we want to take ownership of a vended object, we have to call [object retain] on it, and then it will be there until we explicitly call [object release] or [object autorelease], and if we don’t call release or autorelease on it before we lose our reference to it by changing the variable to something else, we will leak it.

Contrast with [[NSString alloc] initWithFormat:. This method “creates” an object. We own it. Again, it will be there until we explicitly call [object release].