为什么 boost::multi_array 的 ConstMultiArrayConcept 有一个 NumDims 模板参数？

Question

我编写了一个处理

operator<<

的

boost::multi_array

专业化，并使用

ConstMultiArrayConcept

，以便它可以在外部数组和子数组上工作。不过，我想知道为什么

multi_array

概念有一个

std::size_t NumDims

模板参数，因为它可以简单地从

multi_array

中提取。

NumDims

中

ConstMultiArrayConcept

的唯一用途是作为

idgen_helper

的递归深度参数，用于测试切片。

作为参考，以下是

multi_array

概念的标题： http://www.boost.org/doc/libs/1_51_0/boost/multi_array/concept_checks.hpp

这是我超载的

operator<<

template <typename CharT, typename Traits, typename MultiArrayT>
BOOST_CONCEPT_REQUIRES(
                       ((boost::multi_array_concepts::ConstMultiArrayConcept<MultiArrayT, MultiArrayT::dimensionality>)),
                       (std::basic_ostream<CharT, Traits>&)) // return type
operator <<( std::basic_ostream<CharT, Traits>& os, MultiArrayT const& ary )
{
    typename std::basic_ostream<CharT, Traits>::sentry opfx( os );

    if ( opfx ) {
        boost::multi_array_types::size_type const* sizes = ary.shape();
        // using Mathematica array notation
        os << "{";
        for ( int i = 0; i < sizes[0]; ++i ) {
            if ( i > 0 ) os << ", ";
            // verbose just to keep the types apparent
            typedef typename MultiArrayT::const_reference subType;
            subType item = ary[i];
            os << item;
        }
        os << "}\n";
    }
    return os;
}

这个专业化是有效的，但我的理解中肯定缺少一些东西。任何线索将不胜感激。

Answer 1

概念模板类声明：

template <typename Array, std::size_t NumDims>  struct ConstMultiArrayConcept
{
...
};

查看

ConstMultiArrayConcept

在 Boost 代码中实际如何使用：

 template <typename T, std::size_t NumDims>
 class multi_array_ref {
   ...
   // Assignment from other ConstMultiArray types.
   template <typename ConstMultiArray>
   multi_array_ref& operator=(const ConstMultiArray& other)
   {
      function_requires< 
         detail::multi_array::
         ConstMultiArrayConcept<ConstMultiArray,NumDims> >();
      ...

multi_array_view& operator=()

和

sub_array& operator=()

中的代码相同，采用其他

ConstMultiArray

类型。

看起来像

NumDims

不是传递数组类型Array

的维数，它是外部数组类型的维数，用于检查与给定其他数组类型的赋值兼容性。所以无法从 

Array

 模板参数推导出来。

Answer 2

ConstMultiArrayConcept<MA, D>

 和 

MutableMultiArrayConcept<MA, D>

 检查 

MA

 是否满足维度为 

D

 的（可变）数组的概念。

例如，这些概念检查是否成功：

    using MA = boost::multi_array<int, 1>;

    BOOST_CONCEPT_ASSERT((boost::multi_array_concepts::ConstMultiArrayConcept<MA, 1>));
    BOOST_CONCEPT_ASSERT((boost::multi_array_concepts::MutableMultiArrayConcept<MA, 1>));

因为

MA

 的维度为 1。

与

相同

{
    using MA = boost::multi_array<int, 2>;

    BOOST_CONCEPT_ASSERT((boost::multi_array_concepts::ConstMultiArrayConcept<MA, 2>));
    BOOST_CONCEPT_ASSERT((boost::multi_array_concepts::MutableMultiArrayConcept<MA, 2>));
}

然而，这个失败了，因为存在维度不匹配：

{
    using BMA = boost::multi_array<int, 2>;

    BOOST_CONCEPT_ASSERT((boost::multi_array_concepts::ConstMultiArrayConcept<MA, 1>));
    BOOST_CONCEPT_ASSERT((boost::multi_array_concepts::MutableMultiArrayConcept<MA, 1>));

出现错误消息：

/usr/include/boost/multi_array/concept_checks.hpp: In instantiation of ‘static void boost::multi_array_concepts::detail::idgen_helper<0>::call(Array&, const IdxGen&, Call_Type) [with Array = boost::multi_array<int, 2>; IdxGen = boost::detail::multi_array::index_gen<1, 1>; Call_Type = long int]’:
/usr/include/boost/multi_array/concept_checks.hpp:71:44:   required from ‘void boost::multi_array_concepts::ConstMultiArrayConcept<Array, NumDims>::constraints() [with Array = boost::multi_array<int, 2>; long unsigned int NumDims = 1]’
/usr/include/boost/concept/detail/has_constraints.hpp:32:62:   required by substitution of ‘template<class Model> boost::concepts::detail::yes boost::concepts::detail::has_constraints_(Model*, boost::concepts::detail::wrap_constraints<Model, (& Model::constraints)>*) [with Model = boost::multi_array_concepts::ConstMultiArrayConcept<boost::multi_array<int, 2>, 1>]’
/usr/include/boost/concept/detail/has_constraints.hpp:42:5:   required from ‘const bool boost::concepts::not_satisfied<boost::multi_array_concepts::ConstMultiArrayConcept<boost::multi_array<int, 2>, 1> >::value’
/usr/include/boost/concept/detail/has_constraints.hpp:45:51:   required from ‘struct boost::concepts::not_satisfied<boost::multi_array_concepts::ConstMultiArrayConcept<boost::multi_array<int, 2>, 1> >’
/usr/include/boost/concept/detail/general.hpp:51:8:   required from ‘struct boost::concepts::requirement_<void (*)(boost::multi_array_concepts::ConstMultiArrayConcept<boost::multi_array<int, 2>, 1>)>’
/home/user/boost-multi/test/boost_array_concept.cpp:19:2:   required from here
/usr/include/boost/multi_array/concept_checks.hpp:51:8: error: no match for ‘operator[]’ (operand types are ‘boost::multi_array<int, 2>’ and ‘const boost::detail::multi_array::index_gen<1, 1>’)
   51 |       a[ idgen ];
      |       ~^
In file included from /usr/include/boost/multi_array.hpp:34,
                 from /home/user/boost-multi/test/boost_array_concept.cpp:8:
/usr/include/boost/multi_array/multi_array_ref.hpp:530:3: note: candidate: ‘template<int NDims> typename boost::multi_array_ref<T, NumDims>::array_view<NDims>::type boost::multi_array_ref<T, NumDims>::operator[](const boost::detail::multi_array::index_gen<NumDims, NDims>&) [with int NDims = NDims; T = int; long unsigned int NumDims = 2]’
  530 |   operator[](const detail::multi_array::