是什么导致了非类型模板参数的部分专用化中的不同行为？-解网

问：

我正在尝试完成模板元编程练习文本。但是，当我尝试部分专用化非类型模板参数时，我遇到了不同的行为。

我定义了一个编译时向量。

template <int... Nums>
struct Vector {};

然后，我定义一个关于“Vector”的“Get”操作，就可以编译代码了。

template <size_t Idx, typename Vec>
struct Get;

template <int Head, int... Tail>
struct Get<0, Vector<Head, Tail...>> // partial specialize the "Idx"
{
    static constexpr int value = Head;
};

template <size_t Idx, int Head, int... Tail>
struct Get<Idx, Vector<Head, Tail...>>
{
    static_assert(Idx > 0, "Out of Vector bound");
    static constexpr int value = Get<Idx - 1, Vector<Tail...>>::value;
};

但是，当我尝试定义如下所示的“插入”操作时，代码编译在 VS2022 中失败，错误代码为“C2752 多个部分专用化匹配”。

template <size_t Pos, int Val, typename Vec>
struct Insert;

template <size_t Pos, int Val, typename Vec>
using InsertT = typename Insert<Pos, Val, Vec>::type;

template <int Val, int... Elems>
struct Insert<0, Val, Vector<Elems...>> // partial specialize the "Pos"
{
    using type = Vector<Val, Elems...>;
};

template <size_t Pos, int Val, int Head, int... Tail>
struct Insert<Pos, Val, Vector<Head, Tail...>>
{
    using type = PrependT<Head, InsertT<Pos - 1, Val, Vector<Tail...>>>; // push an element in the front of the vector
};

更新：

#include <type_traits>

template <int... Nums>
struct Vector {};

template <int, typename>
struct Prepend;

template <int Ele, typename Vec>
using PrependT = typename Prepend<Ele, Vec>::type;

template <int Ele, int... Nums>
struct Prepend<Ele, Vector<Nums...>>
{
    using type = Vector<Ele, Nums...>;
};

template <size_t Idx, typename Vec>
struct Get;

template <int Head, int... Tail>
struct Get<0, Vector<Head, Tail...>> // partial specialize the "Idx"
{
    static constexpr int value = Head;
};

template <size_t Idx, int Head, int... Tail>
struct Get<Idx, Vector<Head, Tail...>>
{
    static_assert(Idx > 0, "Out of Vector bound");
    static constexpr int value = Get<Idx - 1, Vector<Tail...>>::value;
};

template <size_t Pos, int Val, typename Vec>
struct Insert;

template <size_t Pos, int Val, typename Vec>
using InsertT = typename Insert<Pos, Val, Vec>::type;

template <int Val, int... Elems>
struct Insert<0, Val, Vector<Elems...>> // partial specialize the "Pos"
{
    using type = Vector<Val, Elems...>;
};

template <size_t Pos, int Val, int Head, int... Tail>
struct Insert<Pos, Val, Vector<Head, Tail...>>
{
    using type = PrependT<Head, InsertT<Pos - 1, Val, Vector<Tail...>>>; // push an element in the front of the vector
};

int main()
{
    // test Vector
    static_assert(std::is_same_v<Vector<1, 2>, Vector<1, 2>>);

    // test Prepend
    static_assert(std::is_same_v<Prepend<1, Vector<2, 3>>::type, Vector<1, 2, 3>>);

    // test Get
    static_assert(Get<0, Vector<0, 1, 2>>::value == 0);
    static_assert(Get<1, Vector<0, 1, 2>>::value == 1);
    static_assert(Get<2, Vector<0, 1, 2>>::value == 2);

    // test Insert
    static_assert(std::is_same_v<Insert<0, 3, Vector<4, 5, 6>>::type, Vector<3, 4, 5, 6>>);
    static_assert(std::is_same_v<Insert<1, 3, Vector<4, 5, 6>>::type, Vector<4, 3, 5, 6>>);
    static_assert(std::is_same_v<Insert<2, 3, Vector<4, 5, 6>>::type, Vector<4, 5, 3, 6>>);
    static_assert(std::is_same_v<Insert<3, 3, Vector<4, 5, 6>>::type, Vector<4, 5, 6, 3>>);
}

在我看来，两种操作的部分专业化是相同的。为什么一个编译成功，而另一个编译失败？

C++ 模板元编程部分专用化

基于上述事实，Igor Tandetnik 提出了以下解决方案。此解决方案有两个主要修改。一方面，将更改为，以便和仅在中比较。另一方面，添加了专用化以处理添加到空的情况。Vector<Elems...>Vector<Head, Tail...>Insert<0, Val, Vector<Elems...>>Insert<Pos, Val, Vector<Head, Tail...>>PosValVectorPos=0

template <size_t Pos, int Val, typename Vec>
struct Insert;

template <size_t Pos, int Val, typename Vec>
using InsertT = typename Insert<Pos, Val, Vec>::type;

template <int Val>
struct Insert<0, Val, Vector<>> // Insert element at 0 into an empty vector.
{
    using type = Vector<Val>;
};

template <int Val, int Head, int... Tail>
struct Insert<0, Val, Vector<Head, Tail...>>  // Insert element at 0 into non-empty vector.
{
    using type = Vector<Val, Head, Tail...>;
};

template <size_t Pos, int Val, int Head, int... Tail>
struct Insert<Pos, Val, Vector<Head, Tail...>>
{
    using type = PrependT<Head, InsertT<Pos - 1, Val, Vector<Tail...>>>; // push an element in the front of the vector
};

@Bob__ 通过使用给出了另一种基于 C++20 的解决方案。描述参数的约束。requiresrequiresPos

template <size_t Pos, int Val, typename Vec>
struct Insert;

template <size_t Pos, int Val, typename Vec>
using InsertT = typename Insert<Pos, Val, Vec>::type;

template <int Val, int... Elems>
struct Insert<0ull, Val, Vector<Elems...>> // partial specialize the "Pos"
{
    using type = Vector<Val, Elems...>;
};

template <size_t Pos, int Val, int Head, int... Tail>
  requires (Pos > 0)                                       // <-----
struct Insert<Pos, Val, Vector<Head, Tail...>>
{
    using type = PrependT<Head, InsertT<Pos - 1, Val, Vector<Tail...>>>; // push an element in the front of the vector
};

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是什么导致了非类型模板参数的部分专用化中的不同行为？

What caused the different behaviors in partial specialization of non-type template argument?

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