Calling a C++ DLL which exports a class

Problem/Question/Abstract:

As I stated in an earlier article, it's possible to get an object-reference out from a DLL. This technique is known under the name DLL+. But how about the DLL is written in c++?

Answer:

First of all, you have to translate the header-file (should be delivered with the DLL), which is like an interface-section in ObjectPascal. Headers in c usually contain all sorts of definitions which are relevant outside the
module. In our c++ example it looks like:

/*FILE: income.h */
class CIncome
{
public:
virtual double __stdcall GetIncome( double aNetto ) = 0 ;
virtual void   __stdcall SetRate( int aPercent, int  aYear ) = 0 ;
virtual void   __stdcall FreeObject() = 0 ;
} ;

Then you translate it to an Abstract Class in a unit of her own:

//FILE: income.pas
interface
type
IIncome = class
public
function GetIncome(const aNetto: double): double;
virtual; stdcall; abstract;
procedure SetRate(const aPercent: Integer; aYear: integer);
virtual; stdcall; abstract;
procedure FreeObject; virtual; stdcall; abstract;
end;

In the c++ dll, there is a procedure FreeObject this is necessary because of differences in memory management between C++ and ObjectPascal:

void __stdcall FreeObject()
{
delete this ;
}

When you call the DLL written in C or C++, you have to use the stdcall or cdecl convention. Otherwise, you
will end up in violation troubles and from time to time the application may crash. By the way the DLL, you are calling, should be on the search path;).

So these conventions pass parameters from right to left. With this convention, the caller (that's Delphi)has to remove the parameters from the stack when the call returns.

At least the DLL-call is simple:

incomeRef: IIncome; //member of the reference

function CreateIncome: IIncome;
stdcall; external('income_c.dll');

procedure TfrmIncome.FormCreate(Sender: TObject);
begin
incomeRef := createIncome;
end;

procedure TfrmIncome.btncplusClick(Sender: TObject);
var
cIncome: Double;
begin
// this is the c++ dll+ call ;)
incomeRef.SetRate(strToInt(edtZins.text),
strToInt(edtJahre.text));
cIncome := incomeRef.GetIncome(StrToFloat(edtBetrag.Text));
edtBetrag.text := Format('%f', [cIncome]);
end;


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