声明对另一个 class 的模板化成员数组的模板化成员引用

Question

模板 class OtherClass 有一个模板成员，它是对模板 class BitReferenceHost 模板成员的引用，它是一个 std::array 使用 make_integer_sequence 和两个模板化成员构建的BitReferenceHost 的方法。

我在 BitReferenceHost 中正确声明了 XBitMask，但我在 OtherClass 中遇到了问题，它使用了对 BitReferenceHost 静态成员的引用。

#include <cstdint>
#include <array>
#include <limits>
#include <functional>

template <typename Aspect>
class BitReferenceHost {
    public:

    template <uint8_t BitCount, uint64_t... Indexes>
    static constexpr const std::array<uint64_t, sizeof...(Indexes)>
    initializeXBitMasks(const std::integer_sequence<uint64_t, Indexes...>&) {
        return { initializeXBitMasks<BitCount>( Indexes )... };
    }

    template<
        uint8_t   BitCount,
        uint8_t   ShiftDistance = BitCount + 1,
        uint64_t  BaseMask      = (1ULL << ShiftDistance) - 1
    > static constexpr const uint64_t initializeXBitMasks(
        const uint64_t& mask_index
    ) {
        return BaseMask << (BitCount * mask_index);
    }

    template <
        uint8_t   BitCount,
        typename  MaskInt = uint64_t,
        uint8_t   Count   = std::numeric_limits<MaskInt>::digits / BitCount
    > static constexpr const std::array <MaskInt, Count>
    XBitMask = initializeXBitMasks<BitCount>(
        std::make_integer_sequence<MaskInt, Count>{}
    );

};

template <typename Aspect>
template <uint8_t  BitCount,
  typename MaskInt,
  uint8_t  Count
> const std::array<MaskInt, Count> BitReferenceHost<Aspect>::XBitMask;

template <typename Aspect, typename... Args>
class OtherClass {
    public:
    using ReferenceHost = ::BitReferenceHost<Aspect>;

    template <uint8_t  BitMaskBitCount>
    static constexpr const auto& XBitMask =
            ReferenceHost::template XBitMask<BitMaskBitCount>;
};

template <typename Aspect, typename... Args>
template <uint8_t  BitMaskBitCount>
const auto& OtherClass<Aspect, Args...>::XBitMask;

更新（解决方案）：

max66 提供的每个答案的工作代码：

#include <cstdint>
#include <iostream>
#include <array>
#include <limits>
#include <functional>

template <typename  Aspect> class BitReferenceHost {
  template <uint8_t BitCount, uint64_t... Indexes>
  static constexpr const std::array<uint64_t, sizeof...(Indexes)>
  initializeXBitMasks( const std::integer_sequence<uint64_t, Indexes...>& )
  {
    return { { initializeXBitMasks<BitCount>( Indexes )... } };
  }
  template
  <
    uint8_t   BitCount,
    uint64_t  BaseMask      = (1ULL << BitCount) - 1
  >
  static constexpr uint64_t
  initializeXBitMasks(
    const uint64_t&  mask_index )
  {
    return BaseMask << (BitCount * mask_index);
  }

   public:
  template
  <
    uint8_t   BitCount,
    typename  MaskInt = uint64_t,
    uint8_t   Count   = std::numeric_limits<MaskInt>::digits / BitCount
  >
  static constexpr const std::array<MaskInt,Count>
  XBitMask = initializeXBitMasks<BitCount>( std::make_integer_sequence<MaskInt, Count>{} );
};

template<typename  Aspect>
template
<uint8_t BitCount, typename MaskInt, uint8_t Count>
const std::array<MaskInt, Count>
BitReferenceHost<Aspect>::XBitMask;

template <typename Aspect, typename... Args> class OtherClass
{
  using ReferenceHost = ::BitReferenceHost<Aspect>;
   public:
  template <uint8_t  BitMaskBitCount> static constexpr const decltype(ReferenceHost::template XBitMask<BitMaskBitCount>)&
  XBitMask = ReferenceHost::template XBitMask<BitMaskBitCount>;
};

template <typename Aspect, typename... Args>
template <uint8_t  BitMaskBitCount>
constexpr const decltype(OtherClass<Aspect, Args...>::ReferenceHost::template XBitMask<BitMaskBitCount>)&
OtherClass
<Aspect, Args...>::XBitMask;

int main()
{
  uint64_t test_int = OtherClass<uint64_t>::XBitMask<1>[0];

  std::cout << test_int << std::endl;

  return 0;
}

Answer 1

我不是专家，但是...我在编译您的代码时遇到问题，因为（如果我很好地理解来自 clang 的错误消息）auto 输入

template <typename Aspect, typename... Args>
template <uint8_t  BitMaskBitCount>
const auto& OtherClass<Aspect, Args...>::XBitMask;

无法推导，因为没有初始化值

我用两种方式编译

(1)删除class定义外的声明

template <typename Aspect, typename... Args>
class OtherClass {
    using ReferenceHost = ::BitReferenceHost<Aspect>;

    template <uint8_t  BitMaskBitCount>
    static constexpr const auto& XBitMask =
            ReferenceHost::template XBitMask<BitMaskBitCount>;
};

//template <typename Aspect, typename... Args>
//template <uint8_t  BitMaskBitCount>
//const auto& OtherClass<Aspect, Args...>::XBitMask;

(2) 避免 auto 类型（并使用 using 定义的类型来简化）

template <typename Aspect, typename... Args>
class OtherClass {
    using ReferenceHost = ::BitReferenceHost<Aspect>;
    template <uint8_t  BitMaskBitCount>
    using xbit_t        = decltype(ReferenceHost::template XBitMask<BitMaskBitCount>); 

    template <uint8_t  BitMaskBitCount>
    static constexpr xbit_t<BitMaskBitCount> & XBitMask =
            ReferenceHost::template XBitMask<BitMaskBitCount>;
};

template <typename Aspect, typename... Args>
template <uint8_t  BitMaskBitCount>
constexpr OtherClass<Aspect, Args...>::xbit_t<BitMaskBitCount> & 
   OtherClass<Aspect, Args...>::XBitMask;

声明对另一个 class 的模板化成员数组的模板化成员引用

Declaring templated member reference to templated member array of another class

c++

templates

static-variables

variadic-templates

c++14