2)views::cartesian_product是定制点对象。 以无参数形式调用时,views::cartesian_product()表达式等价于views::single(std::tuple())。 否则,views::cartesian_product(rs...)表达式等价于ranges::cartesian_product_view<views::all_t<decltype((rs))>...>(rs...)。
2)views::cartesian_productis a customization point object. When calling with no argument,views::cartesian_product()isexpression-equivalenttoviews::single(std::tuple()). Otherwise,views::cartesian_product(rs...)isexpression-equivalenttoranges::cartesian_product_view<views::all_t<decltype((rs))>....