demostratse fold expressions introduced C++17 (requires C++20 to run :D)

fold.cpp

#include <format>
#include <iostream>
#include <string>
#include <string_view>

class Factor
{
public:
	Factor(std::string_view representation) :
		m_representation(representation)
	{
	}

	friend Factor operator*(const Factor &a, const Factor &b)
	{
		return Factor(std::format("({} * {})", a.m_representation, b.m_representation));
	}

	friend std::ostream &operator<<(std::ostream &os, const Factor &obj)
	{
		return os << obj.m_representation;
	}

private:
	std::string m_representation;
};

template<typename... Ts>
auto product_left_prime(Ts... a)
{
	return (... * a); // parenthesis is mandatory
}

template<typename... Ts>
auto product_right_prime(Ts... a)
{
	return (a * ...); // parenthesis is mandatory
}

template<typename T, typename... Ts>
auto product_left(T init, Ts... xs)
{
	return (init * ... * xs);
}

template<typename T, typename... Ts>
auto product_right(T init, Ts... xs)
{
	return (xs * ... * init);
}

int main()
{
	std::cout << Factor("a") * Factor("b") * Factor("c") << "\n";
	std::cout << Factor("a") * (Factor("b") * Factor("c")) << "\n";

	std::cout << "\n";

	std::cout << product_left_prime(Factor("a"), Factor("b"), Factor("c")) << "\n";
	std::cout << product_right_prime(Factor("a"), Factor("b"), Factor("c")) << "\n";

	std::cout << "\n";

	std::cout << product_left(Factor("1"), Factor("a"), Factor("b"), Factor("c")) << "\n";
	std::cout << product_right(Factor("1"), Factor("a"), Factor("b"), Factor("c")) << "\n";

	return 0;
}