objective c - Handling NSError when reading from file?

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];