NSKeyedArchiver: How to archive an NSValue containing a custom struct - objective-c

Question

Is there any way I can use an NSKeyedArchiver to encode an NSValue that contains a custom struct?. I have an NSDictionary which contains structs wrapped in NSValues and I want to archive this as part of a larger object graph but I receive the following error:
-[NSKeyedArchiver encodeValueOfObjCType:at:]: this archiver cannot encode structs
However, consider the following:
//Copy of the CGPoint declaration
struct MYPoint {
CGFloat x;
CGFloat y;
};
typedef struct MYPoint MYPoint;
CGPoint point = {1.0, 1.0};
MYPoint myPoint = {1.0, 1.0};
NSLog(#"CGPoint: %s", #encode(CGPoint)); //CGPoint: {CGPoint=ff}
NSLog(#"MYPoint: %s", #encode(MYPoint)); //MYPoint: {MYPoint=ff}
NSValue *CGPointValue = [NSValue valueWithBytes:&point objCType:#encode(CGPoint)];
NSData *CGPointData = [NSKeyedArchiver archivedDataWithRootObject:CGPointValue];
//NO ERROR
NSValue *MYPointValue = [NSValue valueWithBytes:&myPoint objCType:#encode(MYPoint)];
NSData *MYPointData = [NSKeyedArchiver archivedDataWithRootObject:MYPointValue];
//ERROR: -[NSKeyedArchiver encodeValueOfObjCType:at:]: this archiver cannot encode structs
Is it just the case that "this archiver cannot encode your structs" and thats the end of the story or is it possible to get the same behaviour as CGPoint but for custom structs?
I will probably just create a small custom object that wraps an NSValue and implements NSCoding to work around it, but I am curious about the contradiction shown in the code above with CGPoint and wonder if there is a way to extend NSValue to get the same behaviour.

Answer 1

There are issues with something to do with #encode and anonymous structs. Try changing the struct definition to:
typedef struct MYPoint {
CGFloat x;
CGFloat y;
} MYPoint;
If that doesn't work, you can wrap the struct using NSData:
[NSData dataWithBytes:&myPoint length:sizeof(MYPoint)];

Answer 2

The following example illustrates encoding a custom C structure.
//assume ImaginaryNumber defined:
typedef struct {
float real;
float imaginary;
} ImaginaryNumber;
ImaginaryNumber miNumber;
miNumber.real = 1.1;
miNumber.imaginary = 1.41;
NSValue *miValue = [NSValue valueWithBytes: &miNumber withObjCType:#encode(ImaginaryNumber)];
ImaginaryNumber
miNumber2;
[miValue getValue:&miNumber2
];
The type you specify must be of constant length. You cannot store C strings, variable-length arrays and structures, and other data types of indeterminate length in an NSValue—you should use NSString or NSData objects for these types. You can store a pointer to variable-length item in an NSValue object. The following code excerpt incorrectly attempts to place a C string directly into an NSValue object:
/* INCORRECT! */
char *myCString = "This is a string.";
NSValue *theValue = [NSValue valueWithBytes:myCString withObjCType:#encode(char *)];
In this code excerpt the contents of myCString are interpreted as a pointer to a char, so the first four bytes contained in the string are treated as a pointer (the actual number of bytes used may vary with the hardware architecture). That is, the sequence “This” is interpreted as a pointer value, which is unlikely to be a legal address. The correct way to store such a data item is to use an NSString object (if you need to contain the characters in an object), or to pass the address of its pointer, not the pointer itself:
/* Correct. */
char *myCString = "This is a string.";
NSValue *theValue = [NSValue valueWithBytes:&myCString withObjCType:#encode(char **)];
Here the address of myCString is passed (&myCString), so the address of the first character of the string is stored in theValue.
Important: The NSValue object doesn’t copy the contents of the string, but the pointer itself. If you create an NSValue object with an allocated data item, don’t free the data’s memory while the NSValue object exists.