c++ - 折叠可变参数模板检查是否存在成员函数

Question

我正在使用 SFINAE 习惯用法来检查类型是否具有具有特定签名的方法 (function_name())。我实现的解决方案适用于单一类型，但我想让它适用于多种类型(通过可变参数模板)。

template <typename... Other>
class has_foo {
   public:
    static constexpr bool value = has_foo<Other...>::value;
};

template <typename U, typename ...Other> // Error here
class has_foo {
   public:
    static constexpr bool value = has_foo<U>::value && has_foo<Other...>::value;
};

template <typename U>
class has_foo {
   private:
    template <typename T, T>
    struct helper;
    template <typename T>
    static std::uint8_t check(helper<int (*)(size_t), &T::function_name>*);
    template <typename T>
    static std::uint16_t check(...);

   public:
    static constexpr bool value = sizeof(check<U>(0)) == sizeof(std::uint8_t);
};

template <>
class has_foo<void> {
   public:
    static constexpr bool value = false;
};

我收到以下错误:模板重新声明中的模板参数过多。我在这里做错了什么？

Answer 1

template <typename U, typename ...Other>
class has_foo {
public:
    static constexpr bool value = has_foo<U>::value && has_foo<Other...>::value;
};

// partial template specialization
template <typename... Other>
class has_foo<void, Other...>{
public:
    static constexpr bool value = has_foo<void,Other...>::value;
};

// partial template specialization
template <typename U>
class has_foo<U,void> {
private:
    template <typename T, T>
    struct helper;
    template <typename T>
    static std::uint8_t check(helper<int (*)(size_t), &T::function_name>*);
    template <typename T>
    static std::uint16_t check(...);

public:
    static constexpr bool value = sizeof(check<U>(0)) == sizeof(std::uint8_t);
};

// partial template specialization
template <>
class has_foo<void> {
public:
    static constexpr bool value = false;
};