Clang Error on “Potential null dereference.”

Question

I keep getting Clang errors on the following type of code and I can\'t figure out why they\'re erroneous or how to resolve them to Clang\'s satisfaction:

+ (NSSt         


        

Answer 1

The Cocoa convention is that the return value should indicate success or failure (in this case, you return nil for failure) and the error is filled in with additional information, but only when the caller requests it.

In other words

NSError *error = nil;
NSString *result = [self checkForLength: aString error: &error];

and

NSString *result = [self checkForLength: aString error: NULL];

are both valid ways to invoke the method. So the method body should always check for a NULL error param:

if (error != NULL)
    *error = ...;

Answer 2

The way to do what's expected is shown in listing 3-5 in that document. With your example code:

+ (NSString *)checkForLength: (NSString *)theString error: (NSError **)error {
    BOOL hasLength = ([theString length] > 0);
    if (hasLength) return theString;
    else {
        if (error != NULL) *error = [NSError errorWithDomain:@"ErrorDomain" code:hasLength userInfo:nil];
        return nil;
    }
}