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