The power of Objective-c categories.. add category for NSData object

Categories is a very powerful feature in objective-c.. we will show how to use it to overrides the function "description" of the NSData object in the following example:

#import <Foundation/Foundation.h>
#import "../../mem.h"

@interface NSDate(description)
-(NSString*) description;
@end

@implementation NSData(description)
-(NSString*) description
{
    int len = [self length];
    char buff[len];
    [self getBytes: buff];
    
    NSMutableString* ret = [NSMutableString string];
    
    int i;
    for (i=0; i < len; i++)
        [ret appendFormat: @"%i ", buff[i] ];
    
    return ret;
}
@end

int main(void)
{
    POOL;
    char arr[]= {10, 20, 30};
    
    NSData* data = [NSData dataWithBytes:arr length: sizeof(arr)/sizeof(char)];
    
    NSLog(@"%@", [data description]);

    _POOL;
    return 0;
}

Note, POOL and _POOL are two macros defined in "../../mem.h" as follow: #define POOL NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init] #define _POOL [pool drain]