Information about an error condition including a domain, a domain-specific error code, and application-specific information.


@interface NSError : NSObject


Objective-C methods can signal an error condition by returning an NSError object by reference, which provides additional information about the kind of error and any underlying cause, if one can be determined. An NSError object may also provide localized error descriptions suitable for display to the user in its user info dictionary. See Error Handling Programming Guide for more information.

Methods in Foundation and other Cocoa frameworks most often produce errors in the Cocoa error domain (NSCocoaErrorDomain); error codes for the Cocoa Error Domain are documented in the Foundation Constants. There are also predefined domains corresponding to Mach (NSMachErrorDomain), POSIX (NSPOSIXErrorDomain), and Carbon (NSOSStatusErrorDomain) errors.

NSError is “toll-free bridged” with its Core Foundation counterpart, CFErrorRef. See Toll-Free Bridging for more information.

Subclassing Notes

Applications may choose to create subclasses of NSError, for example, to provide better localized error strings by overriding localizedDescription.


Creating Error Objects

+ errorWithDomain:code:userInfo:

Creates and initializes an NSError object for a given domain and code with a given userInfo dictionary.

- initWithDomain:code:userInfo:

Returns an NSError object initialized for a given domain and code with a given userInfo dictionary.

Getting Error Properties


The error code.


A string containing the error domain.


The user info dictionary.

Getting a Localized Error Description


A string containing the localized description of the error.


An array containing the localized titles of buttons appropriate for displaying in an alert panel.


A string containing the localized recovery suggestion for the error.


A string containing the localized explanation of the reason for the error.

Providing Error User Info

+ setUserInfoValueProviderForDomain:provider:

Specifies a block that is called from the implementations of localizedDescription, localizedFailureReason, localizedRecoverySuggestion, localizedRecoveryOptions, recoveryAttempter, and helpAnchor when the underlying value for any of those properties is not present in the userInfo dictionary of NSError instances with the specified domain.

+ userInfoValueProviderForDomain:

Returns any user info provider specified by the setUserInfoValueProviderForDomain:provider: method for a given error domain.


These keys may exist in the user info dictionary.

Getting the Error Recovery Attempter


The object in the user info dictionary corresponding to the NSRecoveryAttempterErrorKey key.


The NSErrorRecoveryAttempting informal protocol provides methods that allow your application to attempt to recover from an error. These methods are invoked when an NSError object is returned that specifies the implementing object as the error recoveryAttempter and the user has selected one of the error’s localized recovery options.

Displaying a Help Anchor


A string to display in response to an alert panel help anchor button being pressed.

Supporting Types

Error Domains


Cocoa errors


POSIX/BSD errors


Mac OS 9/Carbon errors


URL loading system errors


The error domain used by NSError when reporting SOCKS errors.


The error domain used by NSError when reporting SSL errors.

Error Codes

NSError Codes

Error codes in the Cocoa error domain.

Type Methods

+ fileProviderErrorForCollisionWithItem:

Returns a properly formatted error object with a NSFileProviderItemCollisionError error code.


Inherits From