Are Delphi and C++ class VMTs compatible?

Question

I need to call some C++ code from Delphi. The C++ code needs to be able to callback into the Delphi code in return. The example shown here Calling a callback function in Delphi from a C++ DLL works perfectly well. However, instead of passing to the C++ a single Delphi function as a callback, I would like to pass a Delphi object implementing an interface.

Edit: By interface I am referring to the C++ terminology, which is a class with pure virtual functions. This is not necessarily a type defined with the Delphi interface keyword. In other words, the following class defines an interface I would like to call from C++:

ICallable = class 
    procedure callMe stdcall; virtual; abstract;
    procedure CallMeAgain stdcall; virtual; abstract;
end;

The ICallable interface would in turn be implemented in Delphi as follows:

MyCallable = class(ICallable)
   procedure callMe override;
   procedure callMeAgain override;
end;

procedure MyCallable.callMe
begin
   WriteLine('I was called');
end;

procedure MyCallable.callMeAgain
begin
   WriteLine('I was called again');
end;

On the C++ side, which is compiled as a DLL, I want to define the ICallable interface as follows:

class ICallable{
public:
  virtual void callMe()=0;
  virtual void callMeAgain()=0;
}

And export the following DLL function so that it can be called by Delphi:

#define DllExport   extern "C" __declspec( dllexport )

DLLExport bool Callback(ICallable* callable){
   callable->callMe();
   callable->callMeAgain();
   return true;
}

And finally back in Delphi:

function Callback(myCallable: ICallable) : Boolean cdecl; external 'dllname'

Question:

This can only work if C++ and Delphi implement their virtual method tables in the same way. Is this the case?

Are Delphi and C++ class VMTs compatible?

1 Answers