1. 类要实现NSCoding协议
   实现encodeWithCoder和initWithCoder方法

2. 先把类的instance archive成NSData，然后存起来

   TGTest *tt = [TGTest new];
   tt.num = 3;
   NSData *dd = [NSKeyedArchiver archivedDataWithRootObject:tt];
   [[NSUserDefaults standardUserDefaults] setObject:dd forKey:@"objtest"];
   

3. 读数据时，读出NSData，然后unarchive得到instance。

   NSData *dd = [[NSUserDefaults standardUserDefaults] objectForKey:@"objtest"];
   @try
   {
       TGTest *tt= [NSKeyedUnarchiver unarchiveObjectWithData:dd];
   }
   @catch (NSException *exception)
   {
       NSLog(@"%@",exception);
   }
   @finally
   {
   
   }
   

NSUserDefault存储类