using Helpintfs::IHelpSystem gives an error 'abstract class' in C++Builder - delphi

Question

I am using C++Builder and working with sample Delphi code for calling a new help file system called eViewer. When I convert the Delphi Procedure below the variable IHelpSystem is giving the error: variable type 'System::Helpintfs::IHelpSystem' is an abstract class. The error text is below. How can I use the variable IHelpSystem and the function GetHelpSystem in C++Builder?
//Original Delphi
procedure TFrmHelpViewerMain.btnShowTopicClick(Sender: TObject);
var
HelpSystem: IHelpSystem;
begin
GetHelpSystem(HelpSystem);
if assigned(HelpSystem) then
HelpSystem.ShowTopicHelp('topic3', Application.HelpFile);
end;
//My C++Builder code
#include <System.HelpIntfs.hpp>
void __fastcall TForm99::btnShowTopicClick(TObject *Sender)
{
IHelpSystem HelpSystem; //<< Error Here
GetHelpSystem(HelpSystem);
if( Assigned(HelpSystem) ){
HelpSystem->ShowTopicHelp("topic3", Application->HelpFile);
}
}
[bcc64 Error] CBtest_Unit1.cpp(97): variable type 'System::Helpintfs::IHelpSystem' is an abstract class
unknwn.h(114): unimplemented pure virtual method 'QueryInterface' in 'IHelpSystem'
unknwn.h(118): unimplemented pure virtual method 'AddRef' in 'IHelpSystem'
unknwn.h(120): unimplemented pure virtual method 'Release' in 'IHelpSystem'
System.HelpIntfs.hpp(66): unimplemented pure virtual method 'ShowHelp' in 'IHelpSystem'
System.HelpIntfs.hpp(67): unimplemented pure virtual method 'ShowContextHelp' in 'IHelpSystem'
System.HelpIntfs.hpp(68): unimplemented pure virtual method 'ShowTableOfContents' in 'IHelpSystem'
System.HelpIntfs.hpp(69): unimplemented pure virtual method 'ShowTopicHelp' in 'IHelpSystem'
System.HelpIntfs.hpp(70): unimplemented pure virtual method 'AssignHelpSelector' in 'IHelpSystem'
System.HelpIntfs.hpp(71): unimplemented pure virtual method 'Hook' in 'IHelpSystem'
Here are the help pages from the Embaradero website that discuss IHelpSystem.
IHelpSystem
HelpIntfs.GetHelpSystem

Answer 1

As the error message says, IHelpSystem is an abstract class (it has pure virtual methods that are implemented in derived classes), and as such you can't instantiate it directly, like you are trying to do in your C++ code:
IHelpSystem HelpSystem;
You can declare variables of this type only by pointer (IHelpSystem*) or reference (IHelpSystem&). And indeed, the C++ equivalent of the Delphi statement:
var HelpSystem: IHelpSystem;
is:
IHelpSystem* HelpSystem;
But, since IHelpSystem is a Delphi-based interface, you actually need to use the _di_IHelpSystem wrapper type (which is an alias for DelphiInterface<IHelpSystem>), which is what GetHelpSystem() actually outputs in C++:
extern DELPHI_PACKAGE bool __fastcall GetHelpSystem(/* out */ _di_IHelpSystem &System)/* overload */;
Try this instead:
#include <System.HelpIntfs.hpp>
void __fastcall TForm99::btnShowTopicClick(TObject *Sender)
{
_di_IHelpSystem HelpSystem;
GetHelpSystem(HelpSystem);
if (HelpSystem)
HelpSystem->ShowTopicHelp(_D("topic3"), Application->HelpFile);
}