non-type parameter depends on arguments that come after it

Question

I'm writing a templated class that encapsulates win32 handles RAII. This is what I've got so far:

#define NOMINMAX
#include <Windows.h>
#include <functional>

// problem: optionally pass additional params to the deleter function
template<typename ResourceT, std::function<void(ResourceT)> &Deleter>
class Win32Raii 
{
   Win32Raii(const Win32Raii &);
   Win32Raii &operator=(const Win32Raii &);

public:
   Win32Raii()
      : m_resource(nullptr)
   {}

   Win32Raii(const ResourceT &r)
      : m_resource(r)
   {}

   Win32Raii(Win32Raii &&other)
      : m_resource(nullptr)
   {
      *this = std::move(other);
   }

   ~Win32Raii()
   {
      if (m_resource)
      {
         Deleter(m_resource);
      }
   }

   Win32Raii &operator=(Win32Raii &&other)
   {
      std::swap(m_resource, other.m_resource);

      return *this;
   }

   ResourceT get() const { return m_resource; }

private:
   ResourceT m_resource;
};

// library code for each resource type
std::function<void(HICON)>destroy_icon = [](HICON h){ ::DestroyIcon(h); };
std::function<void(HDC) > delete_dc    = [](HDC h){ ::DeleteDC(h); };

//problem: pass real HWND first arg, not just a nullptr constant
std::function<void(HDC) > release_dc = [](HDC dc) { ::ReleaseDC(nullptr, dc); };

typedef Win32Raii<HICON, destroy_icon> HiconRaii;
typedef Win32Raii<HDC,   delete_dc   > HdcDelRaii;
typedef Win32Raii<HDC,   release_dc  > HdcRelRaii;
typedef Win32Raii<HMENU, destroy_menu> HmenuRaii;

//client usage examples
void main()
{
  HWND hWnd = ::FindWindowA(nullptr, "some window");
  // problem: pass hWnd
  HdcRelRaii rdc(::GetDC(hWnd) /*, hWnd */);
  HdcDelRaii ddc(::CreateCompatibleDC(rdc.get()));

  HiconRaii h;
  HiconRaii h2(::LoadIconW(nullptr, IDI_APPLICATION));

  h = HiconRaii(std::move(h2));

  HiconRaii h3 = std::move(h);

  h3 = HiconRaii();
}

It's good for API's that take a single HANDLE argument and release it. Now, my problem is the API's that take multiple arguments in order to release a handle, like

ReleaseDC(HWND, HDC);
SelectObject(HDC, HGDIOBJ);

The client usage syntax I'm willing to achieve in case of multiple parameters to the deleter function is:

HdcRelRaii dc_to_release_in_dtor(::GetDC(hWnd), hWnd);
SelectObjRaii obj_to_reselect_in_dtor(::SelectObject(hBrush, hDC), hDC);

So, finally the question: how can I change

template<typename ResourceT, std::function<void(ResourceT)> &Deleter>

to something variadic like

template<
  typename ResourceT,
  std::function<void(ResourceT, Args&...) &Deleter,
  typename... Args>
>

?

Obviousely, the non-type parameter Deleter depends on arguments that come after it, which is illegal. Hence: stuck...

Thanks in advance

P.S. any ideas for a better title, better usage syntax and so on, are welcome

Answer 1

Here's how it could work (using the make_index_sequence of C++1y). Not sure of your design of using that reference parameter, though

template<typename Signature, std::function<Signature> &Deleter>
class Win32Raii;

template<typename ...Types, std::function<void(Types...)> &Deleter>
class Win32Raii<void(Types...), Deleter> { 
public:
   ~Win32Raii() {
      // do as proposed in http://stackoverflow.com/a/7858971/34509
      callDelete(std::make_index_sequence<sizeof...(Types)>{});
   }

private:
   template<std::size_t ...I>
   void callDelete(std::integer_sequence<std::size_t, I...>) {
       Deleter(std::get<I>(args)...);
   }

   std::tuple<Types...> args;
};

This is how I would change it

template<typename Deleter, typename ...Params>
class Win32Raii {
public:
   Win32Raii(Params... args, Deleter deleter = Deleter());
   ~Win32Raii() { /* copy most from above ... */ }

private:
   Deleter deleter;
   std::tuple<Params...> args;
};

This is completely compatible with a layer of indirection

template<typename Type, Type &Deleter>
struct ExoticDeleter {
   template<typename ...T>
   void operator()(T&&...t) const (
      Deleter(std::forward<T>(t)...);
   }
};

typedef Win32Raii< 
   ExoticDeleter<decltype(destroy_icon), destroy_icon>,
   HICON
> HiconRaii;

I reversed the order, so that I multiple parameters could be given as simply a sequence without wrapping them. But that's just a matter of taste.