NSData 转换

NSString 转换成NSData 对象 

NSData* xmlData = [@"testdata" dataUsingEncoding:NSUTF8StringEncoding];

NSData 转换成NSString对象 

NSData * data; 
NSString *result = [[NSString alloc] initWithData:data  encoding:NSUTF8StringEncoding];

NSData 转换成char* 

NSData *data; 
char *test=[data bytes];

char* 转换成NSData对象 

byte* tempData = malloc(sizeof(byte)*16); 
NSData *content=[NSData dataWithBytes:tempData length:16];