HelpBuilding a wrapper for function-type variables

std::function<void()> function = [instance]( ) { instance->desired_function( [...] ); };

struct tag_stdcall { };
struct tag_thiscall { };
struct tag_cdecl { };

namespace details {

	template<typename _Tag, typename _Base>
	class _Function_wrapper_base
	{
	public:
		using tag_type = _Tag;
		using self_type = _Base;
		using base_type = _Function_wrapper_base<_Tag, _Base>;

	protected:
		_Function_wrapper_base( )
			: f_( nullptr )
		{}

		_Function_wrapper_base( void *f )
			: f_( f )
		{}

		template<typename R, typename..._Args>
		_Function_wrapper_base( R(*f)( _Args... ) )
			: f_( reinterpret_cast<void*>( f ) )
		{}

		_Function_wrapper_base( std::nullptr_t )
			: f_( nullptr )
		{}

		void *__Target( ) 
		{
			return f_;
		}

		void __Target( void *f ) 
		{
			f_ = f; 
		}

		template<typename R, typename..._Args>
		void __Target( R(*f)( _Args... ) )
		{
			f_ = reinterpret_cast<void*>( f );
		}

		void __Target( std::nullptr_t )
		{ 
			f_ = nullptr;
		}

		bool __Empty( )
		{ 
			return f_ == nullptr; 
		}

	private:
		void *f_;
	};

	template<typename _Tag>
	struct is_tag : public std::false_type {};

	template<>
	struct is_tag<tag_stdcall> : public std::true_type {};

	template<>
	struct is_tag<tag_thiscall> : public std::true_type {};

	template<>
	struct is_tag<tag_cdecl> : public std::true_type {};

}

template<typename _Tag>
class function_wrapper
{
	// if _Tag is not tag_stdcall/thiscall/cdecl this will cause a compiler error
	static_assert( details::is_tag<_Tag>::value, "function_wrapper<_Tag> uses unknown _Tag type" );
};


// Template specilization, that is how we make our implementation differ per tag.
template<>
class function_wrapper<tag_stdcall> : public details::_Function_wrapper_base<tag_thiscall, function_wrapper<tag_thiscall>>
{
public:
	function_wrapper( ) 
		: base_type( ) 
	{}

	template<typename _Callable>
	function_wrapper( _Callable &&f )
		: base_type( std::forward<_Callable>( f ) )
	{}

	template<typename R = void, typename..._Args>
	R invoke( _Args&&...args )
	{
		if ( !base_type::__Empty( ) ) // Notice that we put __stdcall as a function specifier
			return (reinterpret_cast<R(__stdcall*)(_Args...)>( base_type::__Target( ) ))( std::forward<_Args>( args )... );
		throw std::exception( "function_wrapper<tag_thiscall>::invoke - empty callable object" );
	}

	void *get_target( ) 
	{ 
		return base_type::__Target( );
	}

	template<typename _Callable>
	void set_target( _Callable &&f )
	{
		base_type::__Target( std::forward<_Callable>( f ) );
	}

};

using stdcall_function = function_wrapper<tag_stdcall>;
using thiscall_function = function_wrapper<tag_thiscall>;
using cdecl_function = function_wrapper<tag_cdecl>;

// not gonna implement the rest, do them yourself!

template<typename _Ty, typename _Ret, typename..._Args>
class member_function
{
public:
	using self_type = member_function<_Ty, _Ret, _Args...>;
	using value_type = typename std::remove_all_extents<_Ty>::type;
	using ptmf_type = _Ret(value_type::*)(_Args...);

	member_function( value_type *instance, ptmf_type ptmf )
		: this_( instance ), mfn_( ptmf ), unit_( false )
	{}

	member_function( std::nullptr_t )
		: mfn_( ), this_( nullptr ), unit_( true )
	{}

	member_function( )
		: mfn_( ), this_( nullptr ), unit_( true )
	{}

	template<typename..._Ax>
	_Ret invoke( _Ax&&...args )
	{
		if ( !unit_ )
			return (this_->*mfn_)( std::forward<_Ax>( args )... );
		throw std::exception( "member_function<_Ty, _Ret, _Args...> empty function" );
	}

	void set_function( std::nullptr_t )
	{ 
		unit_ = true;
	}

	void set_function( value_type *instance, ptmf_type ptmf )
	{
		mfn_ = ptmf;
		this_ = instance;
	}


private:
	value_type *this_;
	ptmf_type mfn_;
	bool unit_;
};