Errordomain

Errordomain is a concept in software engineering referring to the source or category of an error. In many error-handling systems, errors are identified by a pair: a domain (or namespace) and a code. The domain groups related errors and helps determine how to present or handle them, while the code identifies the specific condition within that domain.

In Apple's NSError model, the domain is an NSString attached to an NSError object, distinguishing errors from different subsystems. The code is an NSInteger that identifies the precise condition within the domain. Standard domains include NSCocoaErrorDomain for Cocoa framework errors, NSURLErrorDomain for URL loading errors, NSPOSIXErrorDomain for POSIX errno values, NSOSStatusErrorDomain for OSStatus-based errors, and NSMachErrorDomain for Mach kernel errors. Applications may also define custom domains for their own errors, using a reverse-DNS string like com.example.MyLibraryErrorDomain to avoid collisions.

When handling errors, code typically switches on the domain and the code to determine the appropriate user-facing message or recovery action. The NSError object also carries a userInfo dictionary with keys such as NSLocalizedDescriptionKey, NSLocalizedFailureReasonErrorKey, and NSRecoverySuggestionErrorKey to provide more context.

In Swift, errors are often modeled with the Error protocol, but NSError bridging preserves the domain and code when interacting with Objective-C APIs. Using URLError, Cocoa error codes are accessible via domain constants and enumerations.

Outside Apple's framework, the errordomain concept appears as a general approach: separate namespaces for error codes to avoid ambiguity and to simplify error handling across modules.