Question

I work with Delphi 2006 and I have a complex class named TMyClassTest that have many methods
Some of those methods create nonvisual components and assign event handlers of those components and run methods of those components.

Also I have two classes that implement the same interface like below:

TMyClass1 = class(Class1, Interface1)
   ... //procedures from the Interface1
   procedure MyClass1Proc1;
 end;

TMyClass2 = class(Class2, Interface1)
   ... //procedures from the Interface1
   procedure MyClass2Proc1;
   procedure MyClass2Proc2
 end;

Now I need that TMyClass1 and TMyClass2 to ‘inherit’ the TMyClassTest, too.
Much more … Interface1 must contain (beyond its methods) all the methods from the MyClassTest.
How can I avoid to implement (like copy/paste) on both clases (TMyClass1 and TMyClass2) all the procedures from TMyClassTest ?
I don’t want to keep the same code on three separate places.

Based on Arioch’s comments I created a solution like:
(see http://docwiki.embarcadero.com/RADStudio/XE3/en/Implementing_Interfaces#Implementing_Interfaces_by_Delegation_.28Win32_only.29)

    type
      IMyInterface = interface
        procedure P1;
        procedure P2;
      end;
      TMyImplClass = class
        procedure P1;
        procedure P2;
      end;
      TMyClass1 = class(Class1, IMyInterface)
        FMyImplClass: TMyImplClass;
        property MyImplClass: TMyImplClass read FMyImplClass implements IMyInterface;
        procedure IMyInterface.P1 = MyP1;
        procedure MyP1;
      end;
      TMyClass2 = class(TInterfacedObject, IMyInterface)
        FMyImplClass: TMyImplClass;
        property MyImplClass: TMyImplClass read FMyImplClass implements IMyInterface;
        procedure P3;
        procedure P4;
      end;
    procedure TMyImplClass.P1;
         // ...
    procedure TMyImplClass.P2;
         // ...
    procedure TMyClass1.MyP1;
         // ...
    procedure TMyClass2.P3;
         // ...
    procedure TMyClass2.P4;
         // ...
    var
      MyClass: TMyClass1;
      MyInterface: IMyInterface;
    begin
      MyClass := TMyClass1.Create;
      MyClass.FMyImplClass := TMyImplClass.Create; //Error !!!! FMyImplClass is a read only    property !!!
      MyInterface := MyClass;
      MyInterface.P1;   // calls TMyClass1.MyP1;
      MyInterface.P2;   // calls TImplClass.P2;
    end;

Because I have an error at MyClass.FMyImplClass := TMyImplClass.Create; I tried to create FMyImplClass declaring constructor from TMyClass1 and TMyClass2 but don’t work ok.
Is there some other method to create FMyImplClass ?

Now I tried a solution that seem to work ok. Can there happen some hidden efects?

    type
      IMyInterface = interface
        procedure P1;
        procedure P2;
        procedure CreateFMyImplClass;
      end;
      TMyImplClass = class
        procedure P1;
        procedure P2;
      end;
      TMyClass1 = class(Class1, IMyInterface)
        FMyImplClass: TMyImplClass;
        property MyImplClass: TMyImplClass read FMyImplClass implements IMyInterface;
        procedure IMyInterface.P1 = MyP1;
        procedure MyP1;
        procedure CreateFMyImplClass;
      end;
      TMyClass2 = class(TInterfacedObject, IMyInterface)
        FMyImplClass: TMyImplClass;
        property MyImplClass: TMyImplClass read FMyImplClass implements IMyInterface;
        procedure P3;
        procedure P4;
        procedure CreateFMyImplClass;
      end;
    procedure TMyImplClass.P1;
         // ...
    procedure TMyImplClass.P2;
         // ...
    procedure TMyClass1.MyP1;
         // ...
    procedure TMyClass1.CreateFMyImplClass;
    begin
     FMyImplClass := TMyImplClass.Create;
    end;
    procedure TMyClass2.P3;
         // ...
    procedure TMyClass2.P4;
         // ...
    procedure TMyClass2.CreateFMyImplClass;
    begin
     FMyImplClass := TMyImplClass.Create;
    end;
    var
      MyInterface: IMyInterface;
    begin
      if WantRemote then
         MyInterface := TMyClass1.Create
      else
         MyInterface := TMyClass2.Create;
      MyInterface.CreateFMyImplClass;   // create FMyImplClass ;
      MyInterface.P2;   // calls TImplClass.P2;
    end;

Answer 1

Delphi does not have Scala-like traits or Python-like mixins, nor it support multiple inheritance a la C++.

If you cannot make Class1 and Class2 inherit from TMyClassTest, then perhaps you have to rely on interface delegation: make TMyClassX no more implementing Interface1 directly, but instead add them a field of TMyClassTest and delegate their Interface1 to this field.

I think you’d better

1. move those new common functions into some Interface0 type
2. make Interface1 inherited from Interface0
3. make some TMyClassesBaseCommonTrait class, implementing Interface0
4. make two subclasses TMyClass1InternalEngine(TMyClassesBaseCommonTrait) and TMyClass2InternalEngine(TMyClassesBaseCommonTrait) implementing (in different, TMyClassX-specific ways, the rest of Interface1(Interface0) API
5. have TMyClassX classes internal private field of TMyClass2InternalEngine type doign real implemntation

Google for “delphi interface delegation” shows this as top link: Delphi: How delegate interface implementation to child object?