C++，多次重复一个函数的输入

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 .