How do you insert a transformed integer_sequence (or similar since I am targeting C++11) into a
variadic template argument?
For example I have a class that represents a set of bit-wise flags (shown below). It is made using a nested-class because you cannot have two
variadic template arguments for the same class. It would be used like typedef Flags<unsigned char, FLAG_A, FLAG_B, FLAG_C>::WithValues<0x01, 0x02, 0x04> MyFlags. Typically, they will be used with the values that are powers of two (although not always, in some cases certain combinations would be made, for example one could imagine a set of flags like Read=0x1, Write=0x2, and ReadWrite=0x3=0x1|0x2). I would like to provide a way to do typedef Flags<unsigned char, FLAG_A, FLAG_B, FLAG_C>::WithDefaultValues MyFlags.
template<class _B, template <class,class,_B> class... _Fs>
class Flags
{
public:
template<_B... _Vs>
class WithValues :
public _Fs<_B, Flags<_B,_Fs...>::WithValues<_Vs...>, _Vs>...
{
// ...
};
};
I have tried the following without success (placed inside the Flags class, outside the WithValues class):
private:
struct _F {
// dummy class which can be given to a flag-name template
template <_B _V> inline constexpr
explicit _F(std::integral_constant<_B, _V>) { } };
// we count the flags, but only in a dummy way
static constexpr unsigned _count = sizeof...(_Fs<_B, _F, 1>);
static inline constexpr
_B pow2(unsigned exp, _B base = 2, _B result = 1) {
return exp < 1 ?
result :
pow2(exp/2,
base*base,
(exp % 2) ? result*base : result);
}
template <_B... _Is> struct indices {
using next = indices<_Is..., sizeof...(_Is)>;
using WithPow2Values = WithValues<pow2(_Is)...>;
};
template <unsigned N> struct build_indices {
using type = typename build_indices<N-1>::type::next;
};
template <> struct build_indices<0> {
using type = indices<>;
};
//// Another attempt
//template < _B... _Is> struct indices {
// using WithPow2Values = WithValues<pow2(_Is)...>;
//};
//template <unsigned N, _B... _Is> struct build_indices
// : build_indices<N-1, N-1, _Is...> { };
//template < _B... _Is> struct build_indices<0, _Is...>
// : indices<_Is...> { };
public:
using WithDefaultValues =
typename build_indices<_count>::type::WithPow2Values;
Of course, I would be willing to have any other alternatives to the whole situation (supporting both flag names and values in the same template set, etc).
I have included a "working" example at ideone: http://ideone.com/NYtUrg - by "working" I mean compiles fine without using default values but fails with default values (there is a #define to switch between them).
Thanks!
