初始化一个constexpr std ::数组，其大小的n维std ::数组

Initialize a constexpr std::array with the size of an n-dimensional std::array

本文关键字：std 数组 初始化 一个 constexpr      更新时间：2023-10-16

我编写了一个函数，该函数采用n维std::array和一个等于输入std::array尺寸的参数包（坐标）。我已经可以估计具有元功能的std ::数组的每个维度的大小，我写了一个函数，计算包装中的参数。

我要1）生成一个新的constexpr std::array，其大小等于输入std::array的尺寸数量。2）阵列应以输入std ::数组的每个维度的大小初始化。有人只用C 11即可填充std::array的技巧。

例如。此代码

using array3d = std::array<std::array<std::array<int, 4>, 4>, 4>;
3d_helper<array3d>(array3d(), 0,0,0);

应生成：

constexpr std::array<int, 3> array = { 4, 4, 4 };

这是我到目前为止所拥有的：

//! Static estimation of std::array container size
// Declare a generic template (which is called initially)
template <size_t dim, class Array>
struct static_size;
// specialization for std::array and first dimension
// creates a struct with a static member "value = N"
template <class T, size_t N>
struct static_size<0, std::array<T, N>> : std::integral_constant<size_t, N> {};
// specialization for std::array and dimension > 0 -> recurse down in dim
template <size_t dim, class InnerArray, size_t N>
struct static_size<dim, std::array<InnerArray, N>> : static_size<dim - 1, InnerArray> {};
template <class FIRST, class... OTHER>
size_t num_args() {
    return 1 + num_args<OTHER...>();
}
template <class FIRST>
size_t num_args() {
    return 1;
}
template <class ARRAY, class... ARGS>
struct 3d_helper {
    static glm::vec3 at_t(const ARRAY &points, ARGS... args) {
        constexpr size_t nargs = num_args<ARGS...>();
        /*
        constexpr size_t n1 = static_size<0, ARRAY>::value - 1;
        constexpr size_t n2 = static_size<1, ARRAY>::value - 1;
        */
        // ...
        using array_t = std::array<size_t, nargs>;
        // fill it somehow
    }   
};

基于我对此相关问题的解决方案，这是一种做到这一点的方法

// Example program
#include <iostream>
#include <string>
#include <array>
// typedefs for certain container classes
template<class T, size_t x>
using array1D = std::array<T, x>;
template<class T, size_t x, size_t y>
using array2D = std::array<std::array<T, y>, x>;
template<class T, size_t x, size_t y, size_t z>
using array3D = std::array<std::array<std::array<T, z>, y>, x>;

template <size_t dim, typename Array>
struct size_of_dim;
// specialization for std array and first dimension
template <typename T, size_t N>
struct size_of_dim<0, std::array<T,N>> : std::integral_constant<size_t, N> {};
// specialization for std array and dimension > 0 → recurse down in dim
template <size_t dim, typename InnerArray, size_t N>
struct size_of_dim<dim, std::array<InnerArray,N>> : size_of_dim<dim-1,InnerArray> {};

template <typename Array>
struct cardinality : std::integral_constant<size_t, 0> {};
template <typename T, size_t N>
struct cardinality<std::array<T,N>> : std::integral_constant<size_t, cardinality<T>::value + 1> {};
template <typename Array>
auto constexpr cardinality_v = cardinality<Array>::value;

template <typename Array, size_t... Ns >
constexpr auto dimensions_impl(std::index_sequence<Ns...>) {
    std::array<size_t, cardinality_v<Array>> result = { size_of_dim<Ns,Array>::value... };
    return result;
}

template <typename Array>
constexpr auto dimensions() {
    return dimensions_impl<Array>(std::make_index_sequence<cardinality_v<Array>>() );
}

int main()
{
    auto test = [](auto arr){
        constexpr auto dims = dimensions<decltype(arr)>();
        for (auto d : dims)
            std::cout << d << ", ";
        std::cout << std::endl;
    };
    test(array1D<float, 1337>());
    test(array2D<float, 7357, 17>());
    test(array3D<float, 3, 4, 5>());
}

demo

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