c++ - 使用 n 维 std::array 的大小初始化 constexpr std::array

Question

我写了一个函数，它接受一个 N 维 std::array 和一个等于输入 std::array 维数的参数包(坐标) >。我已经可以使用元函数估计 std::array 的每个维度的大小，并且我编写了一个函数来计算包中的参数。

我想 1) 生成一个新的 constexpr std::array，其大小等于输入 std::array 的维数。 2) 数组应使用输入 std::array 的每个维度的大小进行初始化。 有人知道如何仅使用 C++11 正确填充 std::array 吗？

例如这段代码

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

应该生成:

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

这是我目前所拥有的:

//! 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
    }   
};

Answer 1

基于我的解决方案 this这里的相关问题是一种方法

// 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