C++, repeat an input for a function multiple times

Question

Is it possible to create an input, which is than repeated N times as a parameter for the function?

An example:

#include <range/v3/view/indices.hpp>
#include <range/v3/view/cartesian_product.hpp>

template<std::size_t length, std::size_t N>
constexpr auto tensor_cartesian_product(){

    const auto cart_input1 = ranges::view::indices(length); //create input

    return ranges::view::cartesian_product(cart_input1, cart_input1, ... /* N times cart_input1 */);
}

Answer 1

You can use pack expansion:

template<std::size_t length, std::size_t... is>
constexpr auto tensor_cartesian_product(std::index_sequence<is...>) {
    const auto cart_input = ranges::view::indices(length);
    return ranges::view::cartesian_product((is, cart_input)...);
}

template<std::size_t length, std::size_t N>
constexpr auto tensor_cartesian_product() {
    return tensor_cartesian_product<length>(std::make_index_sequence<N>{});
}

The trick here is to harness the comma operator:

The comma operator expressions have the form: E1 , E2.

In a comma expression E1, E2, the expression E1 is evaluated, its result is discarded ... . The type, value, and value category of the result of the comma expression are exactly the type, value, and value category of the second operand, E2. ...

The pack (is, cart_input)... will be expanded into (0, cart_input), (1, cart_input), ..., (N - 1, cart_input), and the result of evaluation of each of N terms will be cart_input.