std::array 的 std::array 是否具有连续内存? [英] Does std::array of std::array have contiguous memory?

问题描述

似乎，我找到了如何在 2 行代码中轻松获得具有连续内存的普通 2D 数组:

Seems, that I found how to easily get normal 2D Array with contiguous memory in 2 lines of code:

template<int N, int M>
using Array2D = array<array<int, M>, N>;

让我们解决在Array2D(一点点c++17)中交换最小值和最大值的简单任务:

Let's solve easy task of swapping min and max in Array2D (a little of c++17):

template<int N, int M>
void printArray2D(const Array2D<N, M> &arr);

int main() {
    const int N = 5;
    const int M = 5;
    Array2D<N, M> arr;

    // random init of Array2D 
    generate(arr.front().begin(), arr.back().end(), []()->int {
                                                        return rand() % 100;
                                                    });

    printArray2D(arr);

    auto[a, b] = minmax_element(arr.front().begin(), arr.back().end());

    cout << "Swap minimum and maximum: " << *a << " " << *b << endl << endl;

    iter_swap(a, b);
    printArray2D(arr);

    return 0;
}

template<int N, int M>
void printArray2D(const Array2D<N, M> &arr) {
    for (const auto &row : arr) {
        for (const auto &elem : row) {
            cout << std::setw(3) << elem;
        }
        cout << endl;
        cout << endl;
    }
}

我在 Visual Studio 2017 中得到了下一个结果:

I got next result in Visual Studio 2017:

 41 67 34  0 69

 24 78 58 62 64

  5 45 81 27 61

 91 95 42 27 36

 91  4  2 53 92

Swap minimum and maximum: 0 95

 41 67 34 95 69

 24 78 58 62 64

  5 45 81 27 61

 91  0 42 27 36

 91  4  2 53 92

优点:

• 只需 2 条简单的线即可获得二维数组
• 您通常可以通过 arr[2][2]
访问元素
• 您可以使用 stl 算法

缺点:

• 这个解决方案在调试模式下不能正常工作，我有运行时错误array iterators incompatible
• 我不知道内存是否总是连续分配
• 我不知道它是否适用于其他编译器
• 魔法迭代器

问题:

• Array2D 的连续分配有什么保证吗?
• 以这种方式使用数组迭代器是否符合条件?(不同的迭代器，但要记住指针的连续性和实现)
• 在生产代码中以这种方式(如示例)使用 Array2D 是否安全?如果没有，您能否以最少的代码开销提供解决此任务的良好代码?
• geza:这个answer 与嵌套数组的连续性相矛盾.也许 C++14 中发生了一些变化?

• Is contiguous allocation for Array2D ensured by anything?
• Is it eligible to use array iterators in this way? (different iterators, but bear in mind contiguous and implementation on pointers)
• Is Array2D safe to use in this manner (as in example) in production code? If not, can you present good code for solving this task with minimum code overhead?
• geza: This answer contradicts to continuity of nested arrays. Maybe something has changed in C++14?

推荐答案

根据标准，内存应该是连续的.26.3.7.1 [array.overview] 段落说明(强调我的):

According to the standard the memory should be contiguous. The 26.3.7.1 [array.overview] paragraph states (emphasis mine):

头部定义了一个用于存储固定大小的类模板对象序列.数组是一个连续的容器.一个实例of array 存储 N 个类型为 T 的元素，因此 size() == N 是一个不变的.

The header defines a class template for storing fixed-size sequences of objects. An array is a contiguous container. An instance of array stores N elements of type T, so that size() == N is an invariant.

更新:似乎实现可能包括填充.这些 SO 帖子中有关该主题的更多信息:
std::array的大小是标准定义的吗?
特别是这个答案:
嵌套 std::array 中的数据是否保证连续?

Update: It appears the implementation might include the padding. More info on the subject in these SO posts:
Is the size of std::array defined by standard?
and specifically this answer:
Is the data in nested std::arrays guaranteed to be contiguous?

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