c++ - 从编译时 vector 中删除相邻重复项的元程序

Question

我一直在尝试编写一个元函数来从定义为的编译时 vector 中删除相邻的重复项

template <int...>
struct Vector;

例如。如果输入是:

Vector<1, 2, 2, 3, 3, 4, 5, 5>

输出应该是:

Vector<1, 2, 3, 4, 5>

但是如果输入是:

Vector<1, 2, 2, 3, 4, 4, 1, 5>

输出应该是:

Vector<1, 2, 3, 4, 1, 5>

如果 vector 未排序，则预计会出现重复。

我尝试了以下代码:

#include <iostream>
#include <type_traits>

template <int...>
struct Vector;

template <int I, int... L>
struct Vector<I, L...> {
    int First = I;
};

template <int Elem, typename Vector>
struct Append {
    using type = Vector;
};

template <template<int...> class Vector, int Elem, int... VecArgs>
struct Append<Elem, Vector<VecArgs...>> {
    using type = Vector<Elem, VecArgs...>;
};

template <typename Vector>
struct uniq;

template <template<int...> class Vector, int First, int... Last>
struct uniq<Vector<First, First, Last...>> {
    using type = typename uniq<Vector<Last...>>::type;
};

template <template<int...> class Vector, int First, int... Last>
struct uniq<Vector<First, Last...>> {
    using type = typename Append<First, uniq<Vector<Last...>>::type>::type;
};

template <template<int> typename Vector, int I>
struct uniq<Vector<I>> {
    using type = Vector<I>;
};


int solution(int X) {
    static_assert(std::is_same<uniq<Vector<1, 2, 2, 3, 4, 4>>::type, Vector<1, 2, 3, 4>>::value);
    static_assert(std::is_same<uniq<Vector<1>>::type, uniq<Vector<1>>::type>::value);
    //static_assert(std::is_same<Vector<1, 2, 3>, Append<1, Vector<2, 3>>::type>::value);
    return X;
}

int main() {
    solution(1);
}

我正在尝试递归删除重复项。如果前两个元素相等，则丢弃第一个元素并对其余元素调用 uniq。否则取第一个元素并为剩余元素追加 uniq。

但是这段代码不起作用。产生以下错误。

meta.cpp:32:65: error: type/value mismatch at argument 2 in template parameter list for ‘template<int Elem, class Vector> struct Append’
  using type = typename Append<First, uniq<Vector<Last...>>::type>::type;
                                                                 ^
meta.cpp:32:65: note:   expected a type, got ‘uniq<Vector<Last ...> >::type’
meta.cpp: In function ‘int solution(int)’:
meta.cpp:42:61: error: ‘type’ is not a member of ‘uniq<Vector<1, 2, 2, 3, 4, 4> >’
  static_assert(std::is_same<uniq<Vector<1, 2, 2, 3, 4, 4>>::type, Vector<1, 2, 3, 4>>::value);
                                                             ^~~~
meta.cpp:42:61: error: ‘type’ is not a member of ‘uniq<Vector<1, 2, 2, 3, 4, 4> >’
meta.cpp:42:84: error: template argument 1 is invalid
  static_assert(std::is_same<uniq<Vector<1, 2, 2, 3, 4, 4>>::type, Vector<1, 2, 3, 4>>::value);
                                                                                    ^~
meta.cpp:43:46: error: ‘type’ is not a member of ‘uniq<Vector<1> >’
  static_assert(std::is_same<uniq<Vector<1>>::type, uniq<Vector<1>>::type>::value);
                                              ^~~~
meta.cpp:43:46: error: ‘type’ is not a member of ‘uniq<Vector<1> >’
meta.cpp:43:69: error: ‘type’ is not a member of ‘uniq<Vector<1> >’
  static_assert(std::is_same<uniq<Vector<1>>::type, uniq<Vector<1>>::type>::value);
                                                                     ^~~~
meta.cpp:43:69: error: ‘type’ is not a member of ‘uniq<Vector<1> >’
meta.cpp:43:73: error: template argument 1 is invalid
  static_assert(std::is_same<uniq<Vector<1>>::type, uniq<Vector<1>>::type>::value);
                                                                         ^
meta.cpp:43:73: error: template argument 2 is invalid

我尝试了很多东西。例如。 std::conditional 但似乎无法弄清楚为什么没有任何效果。我是模板元编程的新手，在互联网上找不到很多示例。

如有任何帮助，我们将不胜感激。非常感谢。

Answer 1

1. 您不需要模板模板参数template<int...> class Vector .

2. 当前两个元素相等时，应保留其中一个:

   template <template<int...> class Vector, int First, int... Last>
   struct uniq<Vector<First, First, Last...>> {
       using type = typename uniq<Vector<First, Last...>>::type;
       //                                ^^^^^
   };
   

3. 你也应该处理空包。最简单的方法就是添加 ... :

   template<int... I>
   struct uniq<Vector<I...>> {
       using type = Vector<I...>;
   };
   

在这些更改之后，您的代码将编译并且静态断言将通过。 Demo .

为了完整起见，这里是更正后的代码:

template<int Elem, typename Vector>
struct Append;

template<int Elem, int... VecArgs>
struct Append<Elem, Vector<VecArgs...>> {
    using type = Vector<Elem, VecArgs...>;
};

template<typename Vector>
struct uniq;

template<int First, int... Last>
struct uniq<Vector<First, First, Last...>> {
    using type = typename uniq<Vector<First, Last...>>::type;
};

template<int First, int... Last>
struct uniq<Vector<First, Last...>> {
    using type = typename Append<First, 
        typename uniq<Vector<Last...>>::type>::type;
};

template<int... I>
struct uniq<Vector<I...>> {
    using type = Vector<I...>;
};