How to initwithCoder and encodewithCoder to get value of pointer

Question

It appears that my initwithCoder and encodewithCoder is working, but I'm doing something stupid.

For simplicity, I'm showing the relevant code for my question. I have several objects that get NSData from a pointer and a length, so the values are being stored as a const char * and a length.

Here is the interface:

@interface IDImage : NSObject <NSCoding>    
@property (nonatomic) const char *templateData;
@property (nonatomic) NSUInteger templateSize;
- (void)encodeWithCoder:(NSCoder *)coder;
- (id)initWithCoder:(NSCoder *)coder;

Here is the implementation:

- (void)encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeBytes:(const unsigned char*)self.templateData length:self.templateSize forKey:@"templateData"];
    [encoder encodeInteger:self.templateSize forKey:@"templateSize"];
}

- (id)initWithCoder:(NSCoder *)decoder {
    NSUInteger length = 0;
    self = [super init];
    if (self) {
        self.templateData = (const char *)[decoder decodeBytesForKey:@"templateData" returnedLength:&length];
        self.templateSize = [decoder decodeIntegerForKey:@"templateSize"];
    }
    return self;
}

I think I'm writing out the encoded bytes (assuming that the NSCoder is writing out first char all the way to the end, which is templateSize). But when I try to decode, I retrieve a pointer to templateData. How do I get the value? And if I have the value, will it only contain the first char? Meaning I have a char and a length but not the actual data that I meant to decode in bytes.

Can somebody please help me understand how to encode and decode data in bytes when my object contains only the first char and length?

Also, I stepped through my initWithCoder and encodeWithCoder and the values were the same (as expected) but in the calling code the values are different: templateToRead = [NSKeyedUnarchiver unarchiveObjectWithFile:[documentsDirectory stringByAppendingPathComponent:item]]; — Patricia

Duncan C Duncan C · Accepted Answer · 2014-06-17T21:08:24

If you look at the docs of the related method decodeBytesWithReturnedLength:, it says:

If you need the bytes beyond the scope of the current @autoreleasepool block, you must copy them.

I assume that decodeBytesForKey:returnedLength: follows the same rules. So your code should look like this:

- (id)initWithCoder:(NSCoder *)decoder 
{
  NSUInteger length = 0;
  self = [super init];
  if (self) 
  {
    void* temp = decoder decodeBytesForKey:@"templateData" returnedLength:&length];
    void *bytes = malloc(length);
    memcpy(bytes, temp, length);
    self.templateData = bytes;
    self.templateSize = [decoder decodeIntegerForKey:@"templateSize"];
  }
  return self;
}

Then add code like this to your dealloc:

- (void) dealloc;
{
  if (_templateData)
    free(_templateData);
)

Tip-o-the-hat to @nil, who found the (rather hidden) info in the docs about the temporary lifespan of the returned buffer.

How to initwithCoder and encodewithCoder to get value of pointer

1 Answers