Question

Sorry for the poor title,I’m new to OOP so I don’t know what is the term for what I need to do.

I have, say, 10 different Objects that inherit one Object.They have different amount and type of class members,but all of them have one property in common – Visible.

type TSObject=class(TObject);
  protected
    Visible:boolean;
  end;

type
  TObj1=class(TSObject)
  private 
    a:integer;
    ...(More members)
end;
  TObj2=class(TSObject)
  private 
    b:String;
    ...(More members)
end;

...(Other 8 objects)

For each of them I have a variable.

var Obj1:TObj1;
    Obj2:TObj2;
    Obj3:TObj3;
    ....(Other 7 objects)

Rule 1: Only one object can be initialized at a time(others have to be freed) to be visible.

For this rule I have a global variable

var CurrentVisibleObj:TSObject; //Because they all inherit TSObject

Finally there is a procedure that changes visibility.

procedure ChangeObjVisibility(newObj:TSObject);
begin
  CurrentVisibleObj.Free; //Free the old object
  CurrentVisibleObj:=newObj; //assign the new object
  CurrentVisibleObj:= ??? //Create new object
  CurrentVisibleObj.Visible:=true; //Set visibility to new object
end;

There is my problem,I don’t know how to initialize it,because the derived class is unknown(TObj1,TObj2,Tobj3…Which one?).

How do I do this?

I simplified the explanation,in the project there are TFrames each having different controls and I have to set visible/not visible the same way(By leaving only one frame initialized).

Sorry again for the title,I’m very new to OOP.

Answer 1

One of the first problem here is that you seem to assume you can pass an uninitialized variable to ChangeObjVisibility.

ChangeObjVisibility(Obj3);

Here, if Obj3 is nil(or worse, a dangling pointer), ChangeObjVisibility has no way to know what is the type of the object it needs to create.

One of the way you could get the class of frame to create is with an array of const, or a function with a case.

type
  TSObjectClass = class of TSObject;
const
  ObjectClasses = array[0..X] of TSObjectClass = (TObj1, TObj2, TObj3, ...)

function GetFrameclass(Index : Integer) : TSObjectClass;
begin
  Result := ObjectClasses[Index]

  OR 

  case Index of
    0 : Result := TObj1;
    1 : Result := TObj2;
    (...)
  end;
end;

That will work if the frame doesn’t need any kind of special initialization.

Next, you could have a call like this one :

procedure ChangeCurrentFrame(NewFrameIndex : Integer);
var FrameClass : TSObjectclass;
    vFrame : TSObject;
begin
  FrameClass := GetFrameClass(NewFrameIndex);
  if CurrentVisibleObj.ClassType <> FrameClass then
  begin
    vFrame := FrameClass.Create(nil);
    SetCurrentFrame(vFrame);
  end;
end;

procedure SetCurrentFrame(newObj:TSObject); 
begin 
  if Assigned(CurrentVisibleObj) then
    CurrentVisibleObj.Free; //Free the old object 
  CurrentVisibleObj:=newObj; //assign the new object 
  if Assigned(CurrentVisibleObj) then
    CurrentVisibleObj.Visible:=true; //Set visibility to new object 
end; 

Here, SetCurrentFrame replace you ChangeObjVisibility(What you really do here is change the current frame, changing the visibility is just a “side effect”)