在可变参数模板参数中混合类型和非类型? [英] Mixing types and nontypes in variadic template parameters?

问题描述

是否可以在可变参数模板参数中混合类型和非类型?如果我将一个 std::array 例如作为参数 T 传递给这个类，我还需要传递数组的类型和长度，但是我在下面尝试的方式在遇到值时会导致错误，因为它只需要 Types 的类型:

Is it possible to do mixing of types and nontypes in variadic template parameters? If I were to pass a std::array for instance to this class as parameter T, I would need to also pass a type for the array and a length, but the way I tried it below causes an error when encountering a value, because it only expects types for Types:

template <
    template<class, std::size_t>  class T,
    class ... Types>
class C {

    T<Types...> storage;
};

int main(){
    C<std::array, int, 3> c;
}

错误信息:

error: template argument for template type parameter must be a
      type
    Container<std::array, int, 3> c;
                               ^

有没有办法在可变参数上下文中传递类型和值?

Is there a way to pass types and values in a variadic context?

推荐答案

如我所见，您已经对 T 类必须作为模板参数的参数的数量和类型进行了硬编码.此处不需要可变参数模板.只需这样做:

As I see, you alreadty hardcoded the number and types of parameter the class T must take as template parameter. You don't need variadic templates here. Just do this instead:

template <
    template<class, std::size_t>  class T,
    class A, std::size_t N>
class C {

    T<A, N> storage;
};

int main(){
    C<std::array, int, 3> c; // works!
}

如果你想使用可变参数模板，那么也把它放在模板模板参数中:

If you wish to use variadic templates, then put it in the template template parameter too:

template <
    template<typename...>  class T,
    typename... Types>
class C {

    T<Types...> storage;
};

如果您希望使用该版本但仍想使用 std::array，您可以为已经具有大小的 std::array 创建别名:

If you wish to use that version but still want to use std::array, you can create an alias to std::array that already has a size:

template<typename T>
using array3 = std::array<T, 3>;

C<array3, int> c;

或者，您也可以创建某种模板模板别名，让您选择大小:

Alternatively, you can also create some sort of template template alias that let you choose the size:

template<std::size_t n>
struct sized_array {
    template<typename T>
    using array = std::array<T, n>;
};

C<sized_array<5>::array, int>;

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