On Jul 3, 2008, at 2:57 PM, Chris Paveglio wrote:
My code is like this:
NSMutableString *theSettings;
There's the cause of the warning. NSMutableString is a subclass of
NSString, which means every NSMutableString is an NSString, but not
vice versa. The stringByAppendingPathComponent: method returns an
NSString. The string it returns might theoretically be an
NSMutableString, but there's no guarantee of that -- all you know is
that it's an NSString -- hence the warning. In some languages this
would have been an error and your compilation would have failed, but
Objective-C is more permissive.
In your code you don't actually need theSettings to be a mutable
string. You should change the declaration to
NSString *theSettings;
Better yet, move the declaration inside the loop, as I'll get to below.
The reason your code "worked" is that you didn't do anything with
theSettings that required it to actually be a mutable string.
theSettings = [[NSMutableString alloc] init];
Here you are creating an instance of NSMutableString and assigning it
to theSettings. Note this is still okay if you declare theSettings as
an NSString, because as I said, an NSMutableString is an NSString.
Object inheritance works like this:
NSObject (NSObject is a root class)
NSString (NSString is a subclass of NSObject, so every NSString
is an NSObject)
NSMutableString (similarly, every NSMutableString is an
NSString)
More to the point, you don't need the string you just created. More
on this below.
//myPrefs is an array of strings, each item is like "Library/Safari"
int i;
for (i = 0; i < 8; i++
{
theSettings = [NSHomeDirectory() stringByAppendingPathComponent:
[myPrefs objectAtIndex:i]];
....
}
Thinking about it, do I need the alloc and init commands? Sometimes
I am unsure about what needs alloc or init versus what I can just
declare as a variable without doing that.
You use alloc and init to create a new instance of a class. You don't
need the instance you created, so there was actually no reason to
create it. (In fact, it leaks memory, but that's a separate topic
that you do need to learn, but I'm not going to get into it right
now.) You're only using theSettings as a temporary variable inside
the loop, to hold the result of each string concatenation.
As it happens, you can declare the variable and assign it a value at
the same time. So what I would do is remove the declaration of
theSettings that is outside the loop, and change the statement inside
the loop to this:
NSString *theSettings = [NSHomeDirectory()
stringByAppendingPathComponent:[myPrefs objectAtIndex:i]];
--Andy
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