Object serialization

Object serialization (or object archival in Objective-C terminology), in computing, is the conversion of a object in object-oriented programming to a regular, architecture-dependent structure such as a stream that can be easily stored in a file or other storage device, or piped to another application, and can be easily reconstructed.

Several programming languages support serialization, such as Smalltalk, Objective-C, Java and .NET.

Objective-C Example

The following Objective-C example demonstrates object serialization achieved by overriding the write: and read: methods in the Object root class (NB this is in the GNU runtime variant of Objective-C, in the NeXT-style runtime, the implementation is near identical).

Sender class

The following code defines a Sender class to demonstrate the construction of an object and the method and result of serialization (or archival, as is usual Objective-C terminology). The Sender class stores the time (as per time in the C standard library), and contains methods to convert the class to a TypedStream.

If we were to compile the class below, and run it, the program would send the stream representing the object the standard output, for example GNU TypedStream 1D@îC¡

Sender.h

#import <objc/Object.h>
#import <stdio.h>
#import <time.h> 

@interface Sender : Object
{
   int current_time;
}

- (id) set;
- (id) send;
- (id) write: (TypedStream *) s;
- (id) read: (TypedStream *) s;
@end

Sender.m

#import "Sender.h"
@implementation Sender 

- (id) set
{
   current_time = time(NULL); //set the time
}

- (id) write: (TypedStream *) s
{
   [super write:s]; // write necessary information in Sender's superclass 
                    // so we can reconstruct it
   objc_write_types(s, "i", &current_time); // write the current_time variable.
                                            // the "i" specifies an integer, multiple variables
                                            // can be specified as per @encode.
   return self;
}

- (id) read: (TypedStream *) s
{
   [super read:s]; // read back the superclass information
   objc_read_types(s, "i", &current_time); // write back the types we need.

   return self;
}

- (id) send
{
   //A convenience class to output the stream to standard output.
   TypedStream *s =objc_open_typed_stream(stdout, OBJC_WRITEONLY);
   [self write:s];
   objc_close_typed_stream(s);
}
@end

int
main(void)
{
   Sender *s = [Sender new];
   [s set]; //set the time
   [s send];

   return 0;
}