问题描述
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documentation到在线上看到的examples,numpy数组的创建基本上是通过数据数组的指针并在步幅上包含细节。但是,在C ++部分中,我并不热衷于使用一维array
并使用一些奇特的索引编制,而是希望使用结构。那让我开始思考放置在连续内存中的(同质)变量是否可以被视为array
的一部分。
我的思路如下。 array
的元素放置在连续内存中。 struct
的元素也按照其声明的顺序连续放置(不涉及填充时)。因此从内存位置来看,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,则可以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,提示我的问题的答案为是。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;
}
这是对的还是我缺少什么?您对此事有何看法? (除此之外,我应该切换到std :: array。) 谢谢您的时间!
解决方法
您不知道变量是否连续放置在内存中。您的源代码可能会像这样看起来,但这已经足够了。
如果希望变量的行为类似于数组,请使用数组。
该语言没有提供使用指针算术从另一个变量中获取变量的定义方式,除非这两个元素都在同一数组中。