Does using interfaces forces to implement interfaces return types?

Question

When using interfaces, are the implementing class methods forced to return interfaces, or is there a way to return objects?

An example will clarify:

in unit EmployeeIntf:

IEmployee = interface(IInterface)
  function CoWorker: IEmployee; // We dont know anything about TEmployee here
end;

in unit Employee:

uses EmployeeIntf;
...

TEmployee = class(TObject, IEmployee)
public
  function CoWorker: IEmployee; // Returns an error if set to TEmployee
end;

Current code stops on a compiler error if the method returns a TEmployee: E2211 Declaration of 'CoWorker' differs of declaration in interface IEmployee (using Delphi 2010)

Is the TEmployee.CoWorker method forced to return an interface, or is there a way to return a TEmployee instead, as far as the TEmployee is a IEmployee?

If only interfaces are allowed in this case, what are the OO design reasons for that?

[Edit]

• As asked by many contributors, I dont need ref counting on TEmployee and would like to isolate the above question from any ref counting consideration.

• The background of this question is a need to use a very limited set of public functions of TEmployee in an external component (in a separate package). I can't simply import the 'Employee' unit in the package because of too many uneeded dependencies in uses sections, so I'm looking for a loosely coupling solution.

Thanks,

Answer 1

You can make the compiler happy by introducing a method resolution clause:

type
  TEmployee = class(TInterfacedObject, IEmployee)
    function IEmployee.CoWorker = IEmployeeCoWorker;
  public
    function IEmployeeCoWorker: IEmployee;
    function CoWorker: TEmployee;
  end;

function TEmployee.IEmployeeCoWorker: IEmployee;
begin
  result := CoWorker;
end;

Of course, this will complicate the code, but if it actually is what you need...

Answer 2

are the implementing class methods forced to return interfaces?

No, but implementations are forced to match the signatures of the interface definitions.

is there a way to return objects?

Yes, if you declare the interface accordingly.

type
  IEmployee = interface
    function CoWorker: TEmployee;
  end;