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How to Handle Delphi Generics in C++

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This topic describes some programming issues that you might encounter when dealing with generics, a feature supported by Delphi.

Delphi generics are exposed to C++ as templates. However, it is important to realize that the instantiations occur on the Delphi side, not in C++. Therefore, you can only use these template for types that were explicitly instantiated in Delphi code. For example, let's declare a simple generic, TList<T>, in Delphi:

 
unit MyUnit;
 
interface
  uses System.Generics.Collections;
 
type
  MyTList<T> = class(TList<T>)
  public
    // Anchors constructor/destructor
    constructor Create;
    destructor Destroy; override;
 
    class procedure Cleanup(var L: MyTList<T>); static;
  end;
 
  // DoubleList: instantiates MyTList<double>
  DoubleList = class(MyTList<double>)
  end;
 
  // StringList: instantiates MyTList<string>
  StringList = class(MyTList<string>)
  end;
 
implementation
 
class procedure MyTList<T>.Cleanup(var L: MyTList<T>);
begin
  L.Free;
end;
 
constructor MyTList<T>.Create;
begin
  inherited;
end;
 
destructor MyTList<T>.Destroy;
begin
  inherited;
end;
 
end.

The interface above is exposed to C++ as the following:

#ifndef DelphiunitHPP
#define DelphiunitHPP
 
#pragma delphiheader begin
#pragma option push
#pragma option -w       // Display all warnings
#pragma option -w-inl   // Functions %s are not expanded inline
#pragma option -w-8111  // Accessing deprecated entity
#pragma option -Vx      // Zero-length empty class member
#pragma pack(push,8)
#include <System.hpp>   // Pascal unit
#include <SysInit.hpp>  // Pascal unit
#include <System.Generics.Collections.hpp>  // Pascal unit
#include <System.Generics.Defaults.hpp> // Pascal unit
#include <System.Types.hpp> // Pascal unit
 
//-- user supplied -----------------------------------------------------------
 
namespace Delphiunit
{
//-- type declarations -------------------------------------------------------
template<typename T> class DELPHICLASS MyTList__1;
// Template declaration generated by Delphi parameterized types is
// used only for accessing Delphi variables and fields.
// Don't instantiate with new type parameters in user code.
template<typename T> class PASCALIMPLEMENTATION MyTList__1 : public System::Generics::Collections::TList__1<T>
{
    typedef System::Generics::Collections::TList__1<T> inherited;
 
public:
    __fastcall MyTList__1(void);
    __fastcall virtual ~MyTList__1(void);
    static void __fastcall Cleanup(MyTList__1<T>* &L);
};
 
 
class DELPHICLASS DoubleList;
class PASCALIMPLEMENTATION DoubleList : public MyTList__1<double>
{
    typedef MyTList__1<double> inherited;
 
public:
    /* {MyUnit}MyTList<System_Double>.Create */ inline __fastcall DoubleList(void) : MyTList__1<double>() { }
    /* {MyUnit}MyTList<System_Double>.Destroy */ inline __fastcall virtual ~DoubleList(void) { }
 
};
 
 
class DELPHICLASS StringList;
class PASCALIMPLEMENTATION StringList : public MyTList__1<System::UnicodeString>
{
    typedef MyTList__1<System::UnicodeString> inherited;
 
public:
    /* {MyUnit}MyTList<System_string>.Create */ inline __fastcall StringList(void) : MyTList__1<System::UnicodeString>() { }
    /* {MyUnit}MyTList<System_string>.Destroy */ inline __fastcall virtual ~StringList(void) { }
 
};
 
 
//-- var, const, procedure ---------------------------------------------------
}   /* namespace Delphiunit */
#if !defined(DELPHIHEADER_NO_IMPLICIT_NAMESPACE_USE) && !defined(NO_USING_NAMESPACE_DELPHIUNIT)
using namespace Delphiunit;
#endif
#pragma pack(pop)
#pragma option pop
 
#pragma delphiheader end.
//-- end unit ----------------------------------------------------------------
#endif  // DelphiunitHPP

C++ code linking with the .obj created from the above Delphi unit can use instances of MyTList__1<double> or MyTList__1<System::String>.

C++ code can use the generics defined in Delphi directly as long as the C++ code limits itself to types for which the generic was instantiated on the Delphi side. For example, since the Delphi unit instantiates MyTList<String> and MyTList<double> we can use these here. However, if we try to use MyTList__1<char> we'll get errors since the Delphi side did not instantiate MyTList<AnsiChar>.

 
void UseDLists()
{
  MyTList__1<double>* dblList = new MyTList__1<double>();
  dblList->Add(1.0);
  dblList->Add(1.5);
  double d = dblList->Items[1];
#ifdef _WIN64
  delete dblList
#else
  MyTList__1<double>::Cleanup(dblList);
#endif
 
  MyTList__1<System::String> *strList = new MyTList__1<System::String>();
  strList->Add("First");
  strList->Add("Second");
  strList->Add("Third");
  assert(strList->Count == 3);
 
  System::String str = strList->Items[0];
  assert(str == "First");
  assert(strList->Items[1] == "Second");
  assert(strList->Items[2] == "Third");
 
  strList->Insert(0, "Inserted");
  assert(strList->Count == 4);
  assert(strList->Items[0] == "Inserted");
  assert(strList->Items[1] == "First");
 
  strList->Reverse();
  assert(strList->Items[0] == "Third");
  assert(strList->Items[1] == "Second");
  assert(strList->Items[2] == "First");
  assert(strList->Items[3] == "Inserted");
 
  assert(strList->Contains("Inserted"));
  assert(!strList->Contains("Not Inserted"));
 
  strList->Sort();
  strList->Remove("Inserted");
 
  assert(strList->Items[0] == "First");
  assert(strList->Items[1] == "Second");
  assert(strList->Items[2] == "Third");
 
#ifdef _WIN64
  delete strList;
#else
  MyTList__1<System::String>::Cleanup(strList);
#endif
}

If C++ code attempts to use a Delphi generic for types that were not instantiated in Delphi, you'll get errors at link time. For example, the following code attempts to use MyTList__1<char> when the Delphi code did not explicitly instantiate MyTList<AnsiChar>:

 
void UseListOfChar()
{
  MyTList__1<char>* charList = new MyTList__1<char>();
  charList->Add('a');
  // …
}

While the code above compiles, the following errors are generated at link time:

[ilink32 Error] Error: Unresolved external 'Delphiunit::MyTList__1<char>::' referenced from CPPUNIT.OBJ
[ilink32 Error] Error: Unresolved external '__fastcall Delphiunit::MyTList__1<char>::MyTList__1<char>()' referenced from CPPUNIT.OBJ
[ilink32 Error] Error: Unresolved external '__fastcall System::Generics::Collections::TList__1<char>::Add(const const char)' referenced from CPPUNIT.OBJ
[ilink32 Error] Error: Unable to perform link

To eliminate the error, you have to make sure that the Delphi code uses the type MyTList<AnsiChar>.

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