Question

I’m new to Objective C and having trouble understanding why I am getting this error. I’ve checked other similar questions, but haven’t been able to resolve the issue.

The error is “-[NSConcreteMutableData base64Decoded]: unrecognized selector sent to instance 0x6e15610”

Here is a snippet of the problem code, where the call to base64Decoded is causing the crash.

#import "DDData.h"

- (NSString *)decodeBase64:(NSString *)input
{
    NSData* dataDecoded = [[input dataUsingEncoding:NSUTF8StringEncoding] base64Decoded];    
    return [NSString stringWithUTF8String:[dataDecoded bytes]];
}

And in DDData.h:

#import <Foundation/Foundation.h>

@interface NSData (DDData)

- (NSData *)base64Decoded;

@end

and DDData.m:

@implementation NSData (DDData)

- (NSData *)base64Decoded
{
     // Excluding function code, as it never gets to here
}

@end

Just a note that the Project has ARC enabled. Any ideas as to what might be the issue here? Thanks.

EDIT: I have adjusted the above code to help debug the error:

 NSData* dataDecoded = [input dataUsingEncoding:NSUTF8StringEncoding];
[dataDecoded base64Decoded];

dataDecoded gets a value from dataUsingEncoding, it is not nil when the call to base64Decoded is made. When I step over to the called to base64Decoded, it crashes.

Answer 1

You get that kind of message if you try to execute an undefined method on an object. Try it like this:

NSData *dataDecoded = [[input dataUsingEncoding:NSUTF8StringEncoding] base64EncodedString];

There is no base64Decoded method as far as I know, but there is base64EncodedString. So when you send the base64Decoded message to your NSData object, it isn’t recognized because it’s simply not there.