Why does [[NSError alloc] init]; in Xcode throw an error?

Question

I have the following code in Xcode :

NSError *error = [[NSError alloc] init];
NSData *urlData=[NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];

And it throws following error in the logs

[NSError init] called; this results in an invalid NSError instance. It will raise an exception in a future release. Please call errorWithDomain:code:userInfo: or initWithDomain:code:userInfo:. This message shown only once.

Maybe, you will tell me that the answer is in the log, but I do not understand how to init NSError.

Roath

Because sendSynchronousRequest will alloc the error by itself. thats why you just give a pointer to the error (*). So the Instanz you created will be never used and is invalid. – Benedikt

Answer 1

You are not allowed to create an NSError instance via -init; use -initWithDomain:code:userInfo: instead or the constructor method +errorWithDomain:code:userInfo:.

In your case it's redundant anyway as that method will create it in the case of error.

This is the normal pattern for using it:

NSError *error = nil;
NSData *urlData=[NSURLConnection sendSynchronousRequest:request
                                      returningResponse:&response
                                                  error:&error];
if (!urlData) {
    NSLog(@"Error: %@", [error localizedDescription]);
    return NO;
}

// Successful

Event

You mean nil not nul I suppose, – Roath

Answer 2

Think about what you are doing. You should initialise an NSError* variable by setting it to nil. You don't even have to do that, the compiler does it for you. Initialising it by creating a new NSError object is nonsense. It's the same nonsense that you often see when beginners write

NSArray* array = [[NSArray alloc] init];
array = ...;

In the case of NSError, Cocoa rightfully tells you that creating an NSError object without any error information is nonsense and therefore a bug. But there is not the slightest need to do this. Actually, it would break the line that you missed out where you check if error == nil.

Spavined

You are not supposed to check for error with if (error != nil) { ... }. You are supposed to check the return value from the method and then use error. – Event

Answer 3

I solved it replacing:

NSError *error = [[NSError alloc]init];

to

NSError *error = nil;

Supervision

Answer 4

The log itself states that you should use errorWithDomain:code:userInfo: or initWithDomain:code:userInfo: in order to resolve this issue.

Use of -[NSError init] is not recommended and it can cause exception in future release.

Example:

    NSError *errMsg = [NSError errorWithDomain:@"domain" code:1001 userInfo:@{  
                      NSLocalizedDescriptionKey:@"Localised details here" }];

Ply

Answer 5

In Swift

I Solved this

 let error : NSError? = nil

Milissa

This will never work: an optional declared with let cannot change its value. And anyway, the issue in the question does not apply to Swift, the method signature is different. This answer is wrong and off-topic. – Propensity

Answer 6

Use the following code for NSError in Swift

let error = NSError(domain: "", code: 101, userInfo: [ NSLocalizedDescriptionKey: "error in download"])

and use this error

  print("Domain : \(error.domain)")
  print("code : \(error.code)")
  print("Description : \(error.localizedDescription)")

Endive