c++ - 检查第 n 个可变参数模板参数是否属于特定类型

Question

我想在 C++11 中实现类似 same_type() 函数的功能，如下所示(这是我迄今为止尝试过的方法，但它无法应对下面提到的用例) . 作用是检查T是否和Args的第n个参数是同一类型，目前n=0应该可以满足我的要求，虽然n=其他有意义的值(value)会更好(如果不是直截了当的话也不是很重要)。

template<typename T, typename... Args>
struct MyClass
{
    // check if T is of the same type of the n-th argument of Args
    bool same_type() {
        // currently, I only check the first argument
        // but if possible, it would be more useful to extend 
        // this function to check the n-th argument
        return std::is_same<T,typename std::tuple_element<0, std::tuple<Args...> >::type>;
    }
};

我已经看过了 this answer ，但它不考虑以下用例。

用例:

1.与引用和const限定符一起使用:

MyClass<int,const int&> c;
// expect to see 1, type of int should match const int&, i.e. reference or const should NOT affect the result
std::cout<<c.same_type();    

2.不提供参数使用:

MyClass<int> c;
// expect to see 0 as there is no variadic argument provided to compare with int
std::cout<<c.same_type();    

Answer 1

我建议开发一个类型特征 isSameNth 如下

template <std::size_t, typename...>
struct isSameNth;

template <std::size_t N, typename T, typename A0, typename ... As>
struct isSameNth<N, T, A0, As...> : public isSameNth<N-1U, T, As...>
 { };

template <std::size_t N, typename T>
struct isSameNth<N, T> : public std::false_type
 { };

template <typename T, typename A0, typename ... As>
struct isSameNth<0U, T, A0, As...> : public std::is_same<
   typename std::remove_reference<T>::type const,
   typename std::remove_reference<A0>::type const>
 { };

在模板静态方法中转换same_type()(模板值为N)

template <typename T, typename... Args>
struct MyClass
 {
   template <std::size_t N>
   static constexpr bool same_type()
    { return isSameNth<N, T, Args...>::value; }
 };

下面是一个完整的例子(符合C++11)

#include <type_traits>

template <std::size_t, typename...>
struct isSameNth;

template <std::size_t N, typename T, typename A0, typename ... As>
struct isSameNth<N, T, A0, As...> : public isSameNth<N-1U, T, As...>
 { };

template <std::size_t N, typename T>
struct isSameNth<N, T> : public std::false_type
 { };

template <typename T, typename A0, typename ... As>
struct isSameNth<0U, T, A0, As...> : public std::is_same<
   typename std::remove_reference<T>::type const,
   typename std::remove_reference<A0>::type const>
 { };

template <typename T, typename... Args>
struct MyClass
 {
   template <std::size_t N>
   static constexpr bool same_type()
    { return isSameNth<N, T, Args...>::value; }
 };

int main ()
 {
   static_assert(
      false == MyClass<int, long, int, short>::template same_type<0U>(), "!");
   static_assert(
      true == MyClass<int, long, int, short>::template same_type<1U>(), "!");
   static_assert(
      false == MyClass<int, long, int, short>::template same_type<2U>(), "!");

   static_assert(
      true == MyClass<int const, int &>::template same_type<0U>(), "!");

   static_assert(
      false == MyClass<int const &>::template same_type<0U>(), "!");
 }