How to convert superclass to subclass in iOS

Question

As the title described below , A superclass ,the name is ZpIOObject

typedef NS_OPTIONS(NSUInteger, ZpIOObjectType) {
      ZpIOObjectType_NONE = 0 , // is no exist file or no judgment object type
      ZpIOObjectType_File ,     // it's a file object
      ZpIOObjectType_Document   // it's a document object
};
@interface ZpIOObject:NSObject
      @property (nonatomic , assign ) ZpIOObjectType fileType ; 
      @property (nonatomic , strong ) NSString *absolutePathString ;

      /**create an object or subobject*/
      +(instancetype) ioObjectWithAbsolutePathString:(NSString *)absolutePathString ;
@end

A subclass , the name is ZpIOFileObject

@interface ZpIOFileObject : ZpIOObject
      - (void)fileDataWithAsynchronize:(void(^)(NSData *fileData))result ;
@end

Another subclass , the name is ZpIODocumentObject

@interface ZpIODocumentObject : ZpIOObject
      @property (nonatomic , strong , readonly) NSArray<ZpIOObject*> *subIOObjects ;
@end

@implementation ZpIODocumentObject
@synthesize subIOObjects = _subIOObjects ;

- (NSArray<ZpIOObject *>*)subIOObjects
{
    if( nil == self.absolutePathString || YES == [[self.absolutePathString trim] isEqualToString:emptyString] ){
        return nil ;
    }
    if( nil != self.error ){
        return nil ;
    }
    // 进行IO操作，获取最新的文件当前层级的子文件、子文件夹，一定是最新数据，但对IO开销大
    NSFileManager *fileManager = [NSFileManager defaultManager] ;
    NSError *error ;
    NSArray<NSString*>* subDirsNameArr = [fileManager contentsOfDirectoryAtPath:self.absolutePathString error:&error] ;
    if( nil == subDirsNameArr || 0 == [subDirsNameArr count] ){
        return nil ;
    }
    for( NSString *subFilePathString in subDirsNameArr ){
        NSMutableString *subFileAbsolutePathString = [[NSMutableString alloc] initWithString:subFilePathString] ;
        [subFileAbsolutePathString appendString:[NSString stringWithFormat:@"/%@",subFilePathString]] ;
        ZpIOObject * ioobject = [ZpIOObject ioObjectWithAbsolutePathString:subFileAbsolutePathString] ;
        switch ( ioobject.fileType ) {
            case ZpIOObjectType_File:{
            ; // How to convert ZpIOObject to ZpIOFileObject ??????
            }
            case ZpIOObjectType_Document:{
            ; // How to convert ZpIOObject to ZpIODocumentObject ??????
            }
            break;
        default:{/*do nothing*/}break ;
        }
    }
    return nil ;
}
@end

You can see this code :

        switch ( ioobject.fileType ) {
            case ZpIOObjectType_File:{
            ; // How to convert ZpIOObject to ZpIOFileObject ??????
            }
            case ZpIOObjectType_Document:{
            ; // How to convert ZpIOObject to ZpIODocumentObject ??????
            }
            break;
        default:{/*do nothing*/}break ;
        }

I want to convert ZpIOObject to ZpIOFileObject/ZpIODocumentObject .

I think you will think is so easy : create ZpIOFileObject/ZpIODocumentObject and assignment . for example:

        ZpIOFileObject *fileObject = [[ZpIOFileObject alloc] init] ;
        fileObject.property = ioobject.property
... do something ...

No ! I don't want this .

I want a superclass convert to subclass , why ? In my opinion , ioObject is a existing object , create(alloc) a new object(for example [[ZpIOFileObject alloc] init]) is Waste memory resources !

I can't bear to waste memory resources ！

So ， My question is :

If superclass is an existing object , How to convert superclass' object to subclass' object ?

Answer 1

Tested for a long time, I am sure, can't do that 。

subclass' object can convert to superclass' object , but superclass' object cannot convert to subclass' object .

for example :

A inheritance ASuper

A *a = [[A alloc] init] ;
ASuper * aSuper = a ; // right 
A *aAnother = (A *) aSuper ; // right 

ASuper * aSuperAnother = [[ASpuer alloc]init] ;
A *a_other = aSuperAnother ; // Error !