可以将连续放置在内存中的变量视为数组的一部分吗？

Question

我正在编写一个使用 C++ 进行计算的应用程序，然后使用 pybind11 将多维结果作为 numpy 数组返回。从pybind的文档和在线看到的示例来看，numpy 数组创建基本上是传递数据数组的指针并包含步幅的详细信息。然而，在 C++ 部分，我并不热衷于使用一维array和使用一些花哨的索引，但我宁愿使用结构。这让我思考是否可以将放置在连续内存中的（同质）变量视为array.

我的思路如下。an 的元素array被放置在连续内存中。a 的元素struct也按其声明的顺序连续放置（当不涉及填充时）。因此，从内存放置的角度来看，以下四个变量声明是相同的，例如，如果我要指向第一个元素的指针，那么我可以通过一次执行一个整数值的步骤来遍历所有元素：

struct struct_array
{
    int elem[4] = {};
};

struct struct_ints
{
    int a = {};
    int b = {};
    int c = {};
    int d = {};
};

// integer matrix of shape 3x4
int one_dim_array[3 * 4] = {};
int two_dim_array[3][4] = {};
struct_array array_of_struct_arrays[3] = {};
struct_ints array_of_struct_ints[3] = {};

这是我的测试代码，表明我的问题的答案是肯定的。它执行一些地址打印、设置和读取元素。

#include <iostream>

struct struct_array
{
    int elem[4] = {};
};

struct struct_ints
{
    int a = {};
    int b = {};
    int c = {};
    int d = {};
};

int main(void)
{
    const int rows = 3;
    const int cols = 4;

    int one_dim_array[rows * cols] = {};
    int two_dim_array[rows][cols] = {};
    struct_array array_of_struct_arrays[rows] = {};
    struct_ints array_of_struct_ints[rows] = {};

    std::cout << sizeof(int) << " is the size of an int in bytes\n";

    std::cout << "\nOne dim array\n";
    for (int i = 0; i < 12; ++i)
    {
        one_dim_array[i] = i;
        std::cout << &one_dim_array[i] << "\n";
    }

    std::cout << "\nTwo dim array\n";
    for (int i = 0; i < rows; ++i)
    {
        for (int j = 0; j < cols; ++j)
        {
            two_dim_array[i][j] = i * cols + j;
            std::cout << &two_dim_array[i][j] << "\n";
        }
    }

    std::cout << "\nArray of struct arrays\n";
    for (int i = 0; i < rows; ++i)
    {
        for (int j = 0; j < cols; ++j)
        {
            array_of_struct_arrays[i].elem[j] = i * cols + j;
            std::cout << &array_of_struct_arrays[i] << " " << &array_of_struct_arrays[i].elem[j] << "\n";
        }
    }

    std::cout << "\nArray of struct ints\n";
    for (int i = 0; i < rows; ++i)
    {
        array_of_struct_ints[i].a = i * cols + 0;
        array_of_struct_ints[i].b = i * cols + 1;
        array_of_struct_ints[i].c = i * cols + 2;
        array_of_struct_ints[i].d = i * cols + 3;

        std::cout << &array_of_struct_ints[i] << " " << &array_of_struct_ints[i].a << "\n";
        std::cout << &array_of_struct_ints[i] << " " << &array_of_struct_ints[i].b << "\n";
        std::cout << &array_of_struct_ints[i] << " " << &array_of_struct_ints[i].c << "\n";
        std::cout << &array_of_struct_ints[i] << " " << &array_of_struct_ints[i].d << "\n";
    }

    for (int i = 0; i < 4; ++i)
    {
        // Maybe using a reinterpret_cast would be more modern
        void *void_p = nullptr;
        switch (i)
        {
        case 0:
            void_p = &one_dim_array;
            std::cout << "\nOne dim array\n";
            break;

        case 1:
            void_p = &two_dim_array;
            std::cout << "\nTwo dim array\n";
            break;

        case 2:
            void_p = &array_of_struct_arrays;
            std::cout << "\nArray of struct arrays\n";
            break;

        case 3:
            void_p = &array_of_struct_ints;
            std::cout << "\nArray of struct ints\n";
        }
        int *int_p = (int *)void_p;
        for (int i = 0; i < 12; ++i)
        {
            std::cout << *(int_p + i) << "\n";
        }
    }

    std::cout << "Hello world!";
    return 0;
}

这是正确的还是我错过了什么？你对这个问题有什么想法？（除此之外，我应该切换到 std::array。）谢谢您的时间！

Answer 1

您不知道您的变量在内存中是连续放置的。您的源代码可能看起来像那样，但仅此而已。

如果你想让你的变量表现得像一个数组，那么使用一个数组。

该语言没有提供使用指针算术从另一个变量到达变量的定义方法，除非两者都是同一数组中的元素。