Function to generate a tuple given a size N and a type T

Question

While trying to reply to this question, I found my self in the need of creating a bunch of parameters for a variadic function on the fly where:

• the number of the parameters is not given
• the types are all the same, but unknown (even if they must be default constructible)

At runtime, the standard containers and a for loop can be used to do that.
Anyway, I'd like to generate a set of parameters at compile time, so as to be able to forward them to a variadic function.
Because of that, a std::tuple seemed the obvious solution.

Here arose the question: given a size N and a default constructible type T at compile time, how can I write a function to generate a tuple of the given size?

I'm looking for something like this:

auto tup = gen<MyType, N>();

On SO is a notable example of a recursive generator based structs but I was struggling with a function based solution and I've not been able to find it anywhere.

Answer 1

A correctly written forwarding function (a la std::apply) should work with std::array<T, N> and anything else that implements the std::tuple_size/std::get interface. That said,

template<size_t, class T>
using T_ = T;

template<class T, size_t... Is>
auto gen(std::index_sequence<Is...>) { return std::tuple<T_<Is, T>...>{}; }

template<class T, size_t N>
auto gen() { return gen<T>(std::make_index_sequence<N>{}); }