[RESOLVED] Cannot convert from 'X' to 'Y'

[Mybowlcut]

Hey.

I'm running into a problem with a class that uses template metaprogramming. I'm not well-versed with the methodology, so I'm a bit confused as to why I'm getting the error.

The screen object I'm trying to create:

#define GAME_SCREEN_TYPES TL6(Image, Button, Slider, \
Dialog_Box, Progress_Bar, Input_Box)

class Game
{
    // ...
private:
    boost::shared_ptr<Screen<GAME_SCREEN_TYPES> > game_screen;
};

Creating the screen object:

Game::Game()
: game_screen(Screen<GAME_SCREEN_TYPES>::Create(STGS.GUI_Dir() / "_game_screen.xml"))
{
}

The error(s):

Error 1 error C2440: 'initializing' : cannot convert from 'Type_List<T,U> *' to 'Screen<Type_List> *' e:\dev\boost\boost\smart_ptr\shared_ptr.hpp 179
Error 2 error C2439: 'boost::shared_ptr<T>:x' : member could not be initialized e:\dev\boost\boost\smart_ptr\shared_ptr.hpp 179

The screen class:

// Type used to mark the end of a typelist.
struct Null_Type
{
};

// The typelist. For nested typelists type U is a typelist again, so one can
// build typelists of any length.
template<typename T, typename U>
struct Type_List
{
   typedef T head; 
   typedef U tail; 
};

// Convenience macros for building typelists.
#define TL1(T1)									    Type_List<T1, Null_Type >
#define TL2(T1,T2)								    Type_List<T1, TL1(T2) >
// ...

template<typename Type_List>
class Screen : public Base_Screen
{
public:
    static boost::shared_ptr<Screen<Type_List> > Create()
    {
        return boost::shared_ptr<Screen<Type_List> >(new Type_List());
    }

    static boost::shared_ptr<Screen<Type_List> > Create(const boost_path& file_name)
    {
        boost::shared_ptr<Screen<Type_List> > ptr(Create());
        ptr->Read(file_name);
        return ptr;
    }

	virtual ~Screen()
	{
		LOG("Screen at " + file_name.file_string() + " cleaned up.");
	}
protected:
    Screen() {}

    Screen(const boost_path& file_name)
	: file_name(file_name)
	{
		Read(file_name);
		LOG("Screen at " + file_name.file_string() + " initialised.");
	}

    // ...
};

Base_Screen:

class Base_Screen
: public GUI_Object
, public ticpp::File_Readable
{
public:
	virtual ~Base_Screen() {}
};

Cheers.

[superbonzo]

the error has not to do with metaprogramming; it's simply saying that you cannot construct a Screen<Type_List> shared pointer from a Type_List pointer:

return boost::shared_ptr<Screen<Type_List> >(new Type_List());

should be

return boost::shared_ptr<Screen<Type_List> >(new Screen<Type_List>());

BTW, using the same name for the Type_List<> template and for the Screen Type_List template parameter it's probably not a good idea ...

[Mybowlcut]

the error has not to do with metaprogramming; it's simply saying that you cannot construct a Screen<Type_List> shared pointer from a Type_List pointer:

return boost::shared_ptr<Screen<Type_List> >(new Type_List());

should be

return boost::shared_ptr<Screen<Type_List> >(new Screen<Type_List>());

BTW, using the same name for the Type_List<> template and for the Screen Type_List template parameter it's probably not a good idea ...

I only mentioned it because I couldn't find the source of the error, so thought it may be due to the metaprogramming code. I see now that I am completely blind!

Cheers.