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：too many template parameters in template redeclaration。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;
};

Answer 2

即使我不能测试它，因为它不是 mvce，也许下面的代码适合你。

template<typename... Args>
class has_foo { };

template <typename U, typename ...Other>
class has_foo<U, Other...>: public has_foo<Other...> {
    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)) && has_foo<Other...>::value;
};

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

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

这里是一个精简版，也可以编译：

#include <iostream>

template<typename... Args>
class has_foo { };

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

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

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

int main() {
    std::cout << has_foo<int, double, char>::value << std::endl;
    std::cout << has_foo<int, double, void>::value << std::endl;
    std::cout << has_foo<>::value << std::endl;
}

基本思想是定义一个可变结构，然后专门用于所有需要的情况，让它工作，除了 void 您看起来感兴趣的情况。

目前，returns true 示例中的最后一个案例，我不确定如何处理这个案例，因为您没有给出任何建议。不管怎样，从上面的例子开始，你可以很容易地根据你的要求修改它。

折叠可变参数模板检查是否存在成员函数

Folding variadic template checking for existence of a member function

c++

sfinae

variadic-templates