I need to generate a tuple of class templates given an integer list as a variadic argument. The result I want to achieve is presented in the example below, but doesn´t compile:
template <unsigned int First, unsigned int... Is>
using remove_first = std::integer_sequence<unsigned int, Is...>;
template <unsigned int... Is, unsigned int Last> // Invalid
using remove_last = std::integer_sequence<unsigned int, Is...>;
template <unsigned int N_prev, unsigned int N>
class A
{};
template <unsigned int... Is>
class B
{
std::tuple<A<remove_last<Is>, remove_first<Is>>...> object;
};
int main()
{
B<3, 4, 6, 2> b; // <<= b.object is of type: std::tuple<A<3,4>, A<4,6>, A<6,2>>
}
Is it possible to achieve in a similar way?
Answer
First of all – variadic pack, in general, must be last – so this will not compile:
template <unsigned int... Is, unsigned int Last> // Invalid using remove_last = std::integer_sequence<unsigned int, Is...>;
But you do not need neither remove_first or remove_last. You need to store these pairs A<_, _> one by one in internal template of helper template. I invented(of course, I have no idea I am the first one) this technique when I wanted to implement flat union in this toy project.
So here the code with comments:
#include <tuple>
#include <type_traits>
template <unsigned int N_prev, unsigned int N>
class A
{};
// The helper type
template <unsigned int... Is>
struct DefineAPairs;
template <unsigned int I1, unsigned I2, unsigned int... Is>
struct DefineAPairs<I1, I2, Is...>
{
// in the following line pair A<I1, I2> is added at the proper
// position in nested DefineAPairs
template <typename ...FirstPairs>
using type = typename DefineAPairs<I2, Is...>
::template type<FirstPairs..., A<I1, I2>>;
// here ^^^^^^^^^
};
template <unsigned int I1, unsigned I2>
struct DefineAPairs<I1, I2>
{
// this is sentinel implementation
template <typename ...FirstPairs>
using type = std::tuple<FirstPairs..., A<I1, I2>>;
};
template <unsigned int... Is>
class B
{
public:
using type = typename DefineAPairs<Is...>::template type<>;
type object;
};
int main()
{
B<3, 4, 6, 2> b; // <<= b.object is of type: std::tuple<A<3,4>, A<4,6>, A<6,2>>
// the proof it works
static_assert(std::is_same<std::tuple<A<3,4>, A<4,6>, A<6,2>>,
decltype(b.object)>::value);
}
And demo.