Custom errors (categorising with NSError)

This lesson covers expressing your own kinds of error with NSError in Objective-C, with the aim of distinguishing them more specifically.

Representing error information with NSError is the usual Objective-C practice. A domain (the error's category name) plus a code tells the kinds apart.

The sample code defines a category name in the constant ValidationErrorDomain, and a checkAge() function passes error information back to the caller through an NSError **error — a pointer to a pointer. That double-pointer form is the standard Objective-C error-handling pattern.

A common early stumble is what NSError ** means. It is the mechanism that lets the function write into the pointer the caller handed over, and it takes a while to feel natural.

In professional work, being able to tell "invalid input" apart from "the network failed" is what lets you write a suitable response to each.

📖 Reference code

NSString *const ValidationErrorDomain = @"ValidationErrorDomain";

BOOL checkAge(NSInteger age, NSError **error) {
    if (age < 0) {
        if (error) {
            *error = [NSError errorWithDomain:ValidationErrorDomain
                                          code:1
                                      userInfo:@{NSLocalizedDescriptionKey: @"Age must be 0 or greater"}];
        }
        return NO;
    }
    return YES;
}

NSError *error = nil;
if (!checkAge(-5, &error)) {
    NSLog(@"ValidationError: %@", error.localizedDescription);
}

✍️ Your code

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🧪 This site can't compile or run Objective-C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).