C++: Creating a variable length tuple based on integral template parameter - c++

Question

I am writing a library that has compile time polymorphic objects templated on unsigned integers,
template<unsigned level> class Foo { ... };
, i.e. Foo<0>, Foo<1>, Foo<2> etc up to a certain number determined by each application . Let's say our application specifies the need for numbers 0, 1,..., n.
I need to be able to construct a tuple of vectors of pointers to these objects: std::tuple<std::vector<Foo<n> *>, std::vector<Foo<n-1> *>, ..., vector<Foo<0> *>> by invoking a variadic template function, schematically:
template <unsigned n>
std::tuple<unsigned n, unsigned... Args> create_vector_tuple<n>()
or something like it.
My intuition tells me that this should be possible to accomplish though I am way out of my depth here.
I would be very grateful if anyone could point me to a way to implement this!

Answer 1

Something like the following:
#include <cstdio>
#include <tuple>
#include <vector>
#include <utility>
template<unsigned level> struct Foo {};
template<unsigned N, unsigned... Ns>
std::tuple<std::vector<Foo<N - Ns>*>...> create_vector_tuple_imp(std::integer_sequence<unsigned, Ns...>) {
return {};
}
template <unsigned n>
auto create_vector_tuple() {
return create_vector_tuple_imp<n>(std::make_integer_sequence<unsigned, n>{});
}
template<class T>
void print() {
std::printf("%s\n", __PRETTY_FUNCTION__);
}
int main() {
auto t = create_vector_tuple<10>();
print<decltype(t)>();
}
Outputs:
void print() [with T = std::tuple<std::vector<Foo<10u>*, std::allocator<Foo<10u>*> >, std::vector<Foo<9u>*, std::allocator<Foo<9u>*> >, std::vector<Foo<8u>*, std::allocator<Foo<8u>*> >, std::vector<Foo<7u>*, std::allocator<Foo<7u>*> >, std::vector<Foo<6u>*, std::allocator<Foo<6u>*> >, std::vector<Foo<5u>*, std::allocator<Foo<5u>*> >, std::vector<Foo<4u>*, std::allocator<Foo<4u>*> >, std::vector<Foo<3u>*, std::allocator<Foo<3u>*> >, std::vector<Foo<2u>*, std::allocator<Foo<2u>*> >, std::vector<Foo<1u>*, std::allocator<Foo<1u>*> > >]