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>.