Call Delphi CLASS exported in DLL from C++ code - c++

Question

i have a problem to use delphi class from C++ code. delphi dll demo that export a function that return an object.
my delphi Dll code is as follow:
library DelphiTest;
// uses part....
type
IMyObject = interface
procedure DoThis( n: Integer );
function DoThat : PWideChar;
end;
TMyObject = class(TInterfacedObject,IMyObject)
procedure DoThis( n: Integer );
function DoThat: PChar;
end;
// TMyObject implementation go here ...
procedure TMyObject.DoThis( n: Integer );
begin
showmessage('you are calling the DoThis methode with '+intToStr(n) +'parameter');
end;
function TMyObject.DoThat: PChar;
begin
showmessage('you are calling the DoThat function');
Result := Pchar('Hello im Dothat');
end;
// exporting DLL function :
function CreateMyObject : IMyObject; stdcall;export;
var
txt : TextFile;
begin
AssignFile(txt,'C:\log.log');
Reset(txt);
Writeln(txt,'hello');
Result := TMyObject.Create;
end;
exports CreateMyObject;
in my C++ project i declared the IMyObject interface as follow :
class IMyObject
{
public:
IMyObject();
virtual ~IMyObject();
virtual void DoThis(int n) = 0;
virtual char* DoThat() = 0;
};
and my main function as follow :
typedef IMyObject* (__stdcall *CreateFn)();
int main()
{
HMODULE hLib;
hLib = LoadLibrary(L"DelphiTest.dll");
assert(hLib != NULL); // pass !!
CreateFn pfnCreate;
pfnCreate = (CreateFn)GetProcAddress((HINSTANCE)hLib, "CreateMyObject");
if (pfnCreate == NULL)
{
DWORD errc = GetLastError();
printf("%u\n", errc); // it gets error 127
}
else{
printf("success load\n");
}
IMyObject* objptr = pfnCreate();
objptr->DoThis(5);
FreeLibrary(hLib);
int in;
scanf_s("%i", &in);
return 0;
}
with this example i got an error at run time when i try to access the exported function. the errors is at line :
IMyObject* objptr = pfnCreate();
can you tell me what is wrong about my example.
and if possible any working example to access Delphi class (in DLL) from C++ code.

Answer 1

The first problem is calling convention of the methods. The Delphi interface uses register which is a Delphi specific calling convention. Use stdcall, for example, for the methods of the interface.
The next problem is in the C++. Your C++ interface must derive from IUnknown. Further, it should not declare a constructor or destructor.
Beyond that your Delphi code exports PWideChar which does not map to char*. It maps to wchar_t*.
Looking further ahead, returning a PChar works fine here because your implementation returns a literal. But more serious code will want to use a dynamically allocated string presumably and at that point your design is flawed.
Do note that you need to be an elevate administrator to create a file at the root of the system drive. So that's yet another potential failure point.
I expect there are other mistakes, but that's all I've found so far.