c++ - 如何解开参数包包装器？

我想在编译时使用std::ratio类型进行计算。我已经写了一个用于参数包的基本函数。但是，为了将ratios保存在其他对象中，我将其放在参数包包装器中。如何解开包装好的参数包，以将其推入函数中？

我的代码如下：

#include <ratio>
#include <functional>
#include <initializer_list>

namespace cx {
    // I need constexpr accumulate.
    // However, the standard library currently doesn't provide it.
    // I therefore just copied the code from
    // https://en.cppreference.com/w/cpp/algorithm/accumulate
    // and added the constexpr keyword.
    template<class InputIt, class T, class BinaryOperation>
    constexpr T accumulate(InputIt first, InputIt last, T init, BinaryOperation op)
    {
        for (; first != last; ++first) {
            init = op(std::move(init), *first); // std::move since C++20
        }
        return init;
    }
}

// wrapper type
template <class...T> struct wrapper { };

// helper to get the value out of the ratio type
template <class T>
struct get_val {
    static constexpr auto value = double(T::num) / double(T::den);
};

// function for calculating the product of a vector type
template <class T>
constexpr auto product(T values) {
    return cx::accumulate(std::begin(values),
                          std::end(values),
                          1,
                          std::multiplies<typename T::value_type>());
}

// my calculation (needs a parameter pack, can't the handle wrapper type)
template <class...T>
struct ratio_product
{
    // this works by wrapping the Ts into an initializer list
    // and using that for the calculation
    static constexpr auto value =
        product(std::initializer_list<double>{get_val<T>::value...});
};

//test
int main() {
    //this works on a parameter pack (compiles)
    static_assert(ratio_product<
        std::ratio<5>, std::ratio<5>, std::ratio<4>
    >::value == 100,"");

    //this should work on a parameter pack wrapper (does not compile)
    static_assert(ratio_product<
        wrapper<
            std::ratio<5>, std::ratio<5>, std::ratio<4>
        >
    >::value == 100,"");
}

最佳答案

这是将参数包解包到任意模板中的可重用方法：

template <class Wrapper, template <class...> class Template>
struct unwrap;

template <class... Ts, template <class...> class Template>
struct unwrap<wrapper<Ts...>, Template> {
    using type = Template<Ts...>;
};

这将产生以下static_assert，它可以满足您的期望：

static_assert(
    unwrap<
        wrapper<std::ratio<5>, std::ratio<5>, std::ratio<4>>,
        ratio_product
    >::type::value == 100, "");