统一类型和非类型模板参数 [英] Unify type and non-type template parameters

问题描述

我有一个类型特征，它检查给定类型是否是给定类模板的实例:

I have a type trait that checks if a given type is an instance of a given class template:

template <template <typename...> class C, typename T>
struct check_is_instance_of : std::false_type { };

template <template <typename...> class C, typename ...Ts>
struct check_is_instance_of<C, C<Ts...>> : std::true_type { };

template <template <typename...> class C, typename T>
struct is_instance_of : check_is_instance_of<C, std::remove_cv_t<T>> { };

不幸的是，这不适用于非类型模板参数，因为它们不会被可变参数模板参数捕获"，所以

Unfortunately, this does not work for non-type template parameters as they are not "captured" by the variadic template parameters, so

is_instance_of<std::integral_constant, std::true_type>

产生编译错误.有什么方法可以编写 is_instance_of 的实现，该实现可用于任意数量的类型和非类型模板参数?

yields a compile-error. Is there any way to write an implementation of is_instance_of that works for an arbitrary number of type and non-type template parameters?

推荐答案

我认为没有一种干净的方法可以做到这一点除非，非类型参数都具有相同的类型，并且你知道它是哪种类型.在这种非常特殊的情况下，可以使用函数重载.

I don't think there is a clean way to do this unless the non-type arguments are all of the same type and you know which type it is. In that very specific case, function overloading can be used.

在任何其他情况下，您最终都会遇到完美转发问题的模板参数版本，在该模板变量版本中，您将必须专门处理每种类型/非类型参数组合.

In any other case, you end up in a template-argument version of the perfect forwarding problem where you would have to specialize for every type/nontype argument combination.

如果您只需要处理同类非模板参数，并且可以猜测类型，则应该可以执行以下操作.您可以为不同的类型重载instance_of(此处仅覆盖int)，但是您必须为每种希望处理的类型显式创建一个实例:

If you only need to address homogeneous non-template arguments and you can guess the type, the following should work. You can overload instance_of for different types (only int is covered here), but you'd have to explicitly create an instance for each type you want to be able to handle:

// variation for non-type parameters, only for uniform parameters with
// known type.
template <typename V, template <V...> class C, typename T>
struct check_is_instance_of_nontype : std::false_type { };

template <typename V, template <V...> class C, V... Values>
struct check_is_instance_of_nontype<V, C, C<Values...>> : std::true_type { };

// this is as in your example
template <template <typename...> class C, typename T>
struct check_is_instance_of : std::false_type { };

template <template <typename...> class C, typename ...Ts>
struct check_is_instance_of<C, C<Ts...>> : std::true_type { };

template <template <typename...> class C, typename T>
struct is_instance_of : check_is_instance_of<C, std::remove_cv_t<T>> { };

template <template <typename...> class C, typename T>
constexpr bool instance_of()
{
    return is_instance_of< C, T>::value;
}

template <template <int...> class C, typename T>
constexpr bool instance_of()
{
    return check_is_instance_of_nontype< int, C, T>::value;
}

template< int... >
struct Duck
{
};

template<typename A, typename B>
struct Swallow
{

};

int main() {
    typedef Duck<1, 2> SittingDuck;
    typedef Swallow< int, int> UnladenSwallow;

    std::cout << instance_of< Duck, SittingDuck>() << instance_of< Swallow, UnladenSwallow>();
    return 0;
}

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