Well it does give the scholarly types yet another bamboozling mechanism < s >.

I must say, though, that I think Mario's example is a good one.


>Hi Mario,
>
>I understand inheritance with sub-classes.
>What I don't understand is why I should have a level of "abstract" classes (that's never used) between VFP's BaseClasses and my "concrete" myBase classes that I use to create objects with. When I need different behavior I create a sub-class from myBase, but when I want a universal change I just modify myBase. What does the "abstract" class accomplish?
>
>
>>Abstract classes area like interfaces. You can provide the definition of a funtion/method/property and OBLIGATE to implement this in a derived class..
>>
>>For example, if you want create a backup solution, but need backup not only normal files, but also Sql Server databases, Outlook configurations, etc.. you can:
>>
>>TAbstractBackup
>> Name:String
>> Directory:String
>> Date:DateTime
>> Prepare() Abstract
>> CanBackup:Boolean Abstract
>> DoBackup Abstract
>> CreateFileName
>> oZip.FileName=Directory+"\"+Name+" "+TRANSFORM(Date)+".zip"
>> oZip.Create
>>End
>>
>>TFileBackup
>> File:String
>> CanBackup
>> RETURN Directory(JustPath(this.File)) //check if the dir exist...
>> Prepare
>> 'Nothing
>> DoBackup
>> CreateFileName
>> oZip.AddFile(this.File)
>>End
>>
>>TDBSqlBackup
>> Server:String
>> DBName:String
>> UserName:String
>> Password:String
>> FFileBak:String
>> CanBacup
>> return oCon.Open("here the connection string")
>> Prepare
>> 'Run the backup in Sql Server a store the .BAK File path
>> FFileBack=...
>> 'Optional Shinrk the database
>> DoBackup
>> CreateFileName
>> oZip.AddFile(this.FFileBak)
>> DELETE FILE (this.FFileBak) //not bloat the harddisk!
>> oCon.Close()
>>End
>>NOTE: Not real code here!
>>
>>So, you can see that backup a file is a diferent thing that backup a database. So, is impractical provide a base code for example to DoBackup because you need perform a complete diferent process for a file and to a database, BUT you need force that DoBackup have implementation code, and in this way, you know that TAbstractBackup NEVER must be instantiate (i think that VFP not enforce this, but Delphi & others yes). This is also the idea with a interface, but a Abstract Class can provide base code if need them (but a interface not have never implementation code). In this way, with a factory pattern and maybe a interface you not need worry if a object to backup is a file or something like, your simply do:
>>
>>For i=1 To oBackupsItem.Count
>> oBackupsItem(i).DoBackup