设置二维指针数组时的无限循环问题

Question

我正在尝试设置一个二维指针数组，但是我在实际分配内存时遇到了问题。每当我尝试将值写入数组时，都会出现访问写入位置错误。

网格.h

class Grid 
{
public:
    Grid();

private:
    int x;
    int y;
    int row;
    int col;
    Grid*** array2D;
    
};

Grid.cpp

Grid::Grid()
{
    row = 32;
    col = 30;

     // Allocate memory to point to.
    array2D = new Grid**[row];
    for (int i = 0; i < row; i++) {
        array2D[i] = new Grid*[col];
        }


    for (int i = 0; i <= row; i++)
    {
        for (int j = 0; j <= col; j++)
        {
            array2D[i][j]->x=6;
            array2D[i][j]->y =5;
            
        }
    }

Unhandled exception at 0x000C1CC8 in Project.exe: 0xC0000005: Access violation writing location 0xCDCDCE29.

Exception thrown at 0x000C1CC8 in Project.exe: 0xC0000005: Access violation writing location 0xCDCDCE29.

Answer 1

在提供的代码中，内存已分配给Grid*** 和Grid**，但没有分配给Grid*，网格本身也从未被初始化。所以在最后一级，指针array[x][y] 持有一个无意义的值，它可能指向一些不相关的地方，所以Access violation 错误！

您应该首先分配并可能初始化最后一级指针，如下所示：

array2D[i][j] = new Grid;

然后才尝试访问其元素，例如：

array2D[i][j]->x=6;
array2D[i][j]->y =5;

也别忘了纠正：

    for (int i = 0; i < row; i++)
    {
        for (int j = 0; j < col; j++)

另一种解决方案是简单地从代码中删除一个* 并放置一个默认构造函数以避免构造循环，如下所示： 网格.h

class Grid
{
public:
    Grid() = default;
    Grid(int);

private:
    int x;
    int y;
    int row;
    int col;
    Grid** array2D;
};

Grid.cpp

Grid::Grid(int)
{
    row = 32;
    col = 30;
    x=0;
    y=0;

     // Allocate memory to point to.
    array2D = new Grid*[row];
    for (int i = 0; i < row; i++) {
        array2D[i] = new Grid[col];
        }

    for (int i = 0; i < row; i++)
    {
        for (int j = 0; j < col; j++)
        {
            (array2D[i][j]).x=6;
            (array2D[i][j]).y =5;
       }
    }
}

示例用法：

int main()
{
    Grid grid(0);
    std::cout<<"the end" << std::endl;
    return 0;
}