c++ - 模板模板参数的参数似乎是非模板类型

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Where and why do I have to put the “template” and “typename” keywords?

(6个答案)


3年前关闭。




以下代码会产生错误，但是我不确定为什么会这样。

#include <utility>
#include <type_traits>
#include <tuple>

template <typename T, template <typename...> class F, typename = void>
struct tuple_subtype_apply {
    using type = typename tuple_subtype_apply<T, F,
        std::make_index_sequence<std::tuple_size<T>::value>>::type;
};

template <typename T, template <typename...> class F, std::size_t... Is>
struct tuple_subtype_apply<T, F, std::index_sequence<Is...>>
{
    using type = F<typename std::tuple_element<Is, T>::type...>;
};

template <typename... Args>
using true_t = std::true_type;

template <template <typename...> class Fn, typename... Ts>
struct prepend
{
    template <typename... Args>
    using fn = Fn<Ts..., Args...>;
};

template <typename T, typename Tup>
using test = typename tuple_subtype_apply<Tup,
    prepend<true_t, T>::fn>::type;

产生的错误:(clang -std = c++ 1z)

<source>:29:24: error: type/value mismatch at argument 2 in template parameter list for 'template<class T, template<class ...> class F, class> struct tuple_subtype_apply'
  prepend<true_t, T>::fn>::type;
                    ^
<source>:29:24: note:   expected a class template, got 'prepend<true_t, T>::fn'

手动将模板参数替换为test编译时不会出错(例如typename tuple_subtype_apply<std::tuple<int, int>, prepend<true_t, int>::fn>::type)。不应该将::: fn用作模板类型？

最佳答案

由于prepend<true_t, T>::fn是dependent name，因此需要在template之前添加fn。

template <typename T, typename Tup>
using test = typename tuple_subtype_apply<Tup,
    prepend<true_t, T>::template fn>::type;

关于c++ - 模板模板参数的参数似乎是非模板类型，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/43463705/