Handling NSError when reading from file? - objective-c

Question

I am just curious if I am doing this right.
NSString *fileContents;
NSError *fileError = nil;
fileContents = [[NSString stringWithContentsOfFile:fileOnDisk
encoding:NSMacOSRomanStringEncoding
error:&fileError] retain];
if(fileError != nil) {
NSLog(#"Error : %#", [fileError localizedDescription]);
}
// Other Code ...
[fileContents release];
.
EDIT (to reflect bbums comments)
.
NSString *fileOnDisk = #"/Users/Gary/Documents/Xcode/RnD/Maya.MEL";
NSError *fileError; // Should this be *fileError = nil;
NSString *fileContents;
int status = 0;
fileContents = [[NSString stringWithContentsOfFile:fileOnDisk
encoding:NSMacOSRomanStringEncoding
error:&fileError] retain];
if(fileContents == nil) {
NSLog(#"FileError: %#", [fileError localizedDescription]);
status = 1;
} else {
NSLog(#"Success : %#", fileContents);
}
// Clean up
[fileContents release];
[pool drain];
return status;
gary

Answer 1

NSError *fileError = nil;
....
if(fileError != nil)
....
That is incorrect. You must not assume anything about the return-by-reference value of fileError until you check whether or not fileContents was nil. Not ever. Setting fileError to nil prior to calling the pass-error-by-reference method does nothing useful.
That is, your code should read (fixed now that I'm no longer running from plane to plane and hopping on WiFi in between connections...):
NSString *fileContents;
NSError *fileError;
fileContents = [[NSString stringWithContentsOfFile:fileOnDisk
encoding:NSMacOSRomanStringEncoding
error:&fileError] retain];
if(fileContents == nil) {
NSLog(#"Error : %#", [fileError localizedDescription]);
// ... i.e. handle the error here more
return ...; // often returning after handling the errors, sometimes you might continue
}
// Other Code ...
[fileContents release];