Calling C++ member functions with the thiscall convention

Question

I have an application written in C++ that loads my DLL that is written in Delphi. The application calls an exported function of the DLL and passes it a pointer to an object of a class that has several member functions that I want to call from my Delphi DLL. I have written a Delphi class equivalent of the C++ class:

CClass = class
    procedure A(param : Integer); virtual; abstract;
    procedure B; virtual; abstract;
end

and the C++ class looks the same

class CClass {
public:
    virtual void A(int) = 0;
    virtual void B() = 0;
};

The member functions use the thiscall calling convention which does not have a Delphi equivalent. This probably means I need to use assembly. I have tried:

function exported_procedure(ptr : CClass) : Integer; cdecl;
begin
    asm
        push ebx;
        push esi;
        mov ecx, ptr;
        push 4;
        call CClass.A;
        pop esi;
        pop ebx;
    end;

    Result := 0;
end

But I get a segmentation fault when the application calls this function. How do I properly call the member function?

Answer 1

The asm looks fine, that's how we call a thiscall interface. But this is a class. I don't think there's any reason to assume CClass.A actually calls void A(int). You'd have to figure out the correct offset. Having a Delphi class won't help I think: you should find out the offsets in C++, add that offset to ptr and call that address.