无法为二维数组对象成员赋值

Question

const int MAP_WIDTH = 100;
const int MAP_HEIGHT = 100;
const int RANDOM_POINTS = 10;
const int seeds = 3;

struct Point
{
    int x;
    int y;
    int tileIndex;
};

Point* points = new Point[RANDOM_POINTS];
Point* mapPoints[MAP_WIDTH][MAP_HEIGHT];

Point* nearestPoint(int x, int y)
{
    Point* ptr;
    ptr = points;
    Point* nearest_point;
    int nearest_dis = 1000;
    int distance;
    for (int i = 0; i < RANDOM_POINTS; i++, ptr++)
    {
        int dx = ptr->x;
        int dy = ptr->y;
        distance = abs(dx - x) + abs(dy - y);
        if (distance < nearest_dis)
        {
            nearest_point = ptr;
            nearest_dis = distance;
        }
    }

    return nearest_point;
}

int main(int argc,char* args[])
{
    srand(seeds);
    int min = 25;
    int max = 75;
    for (int i = 0; i < RANDOM_POINTS; i++)
    {       
        points[i].x = rand() % (max - min + 1) + min;
        points[i].y = rand() % (max - min + 1) + min;
        points[i].tileIndex = rand() & 6;
    }

    std::ofstream mapFile;
    //mapFile.open("res/mapFile.map",std::ios::app);

    for (int px = 0; px < MAP_WIDTH; px++)
    {
        for (int py = 0 ; px < MAP_HEIGHT; py++)
        {
            mapPoints[px][py]->x = px;
            mapPoints[px][py]->y = py;
            mapPoints[px][py]->tileIndex = nearestPoint(mapPoints[px][py]->x, mapPoints[px][py]->y)->tileIndex;
            std::cout << mapPoints[px][py]->x << mapPoints[px][py]->y << std::endl;
            /*if(px<MAP_WIDTH-1)
                mapFile << mapPoints[px][py]->tileIndex << ",";
            else
                mapFile << mapPoints[px][py]->tileIndex << "\n";*/      
        }
    }
    mapFile << std::endl;
    mapFile.close();
    //std::cout << nearestPoint(3, 3)->tileIndex << std::endl;
    return 0;
}

points[RANDOM_POINTS] store random generated points .

mapPoints[MAP_WIDTH][MAP_HEIGHT] is map size .

The purpose of this code is output random generated tile type index to a text file ,so I can use it with tileset.png file .

But I find out the 2d array mapPoints[MAP_WIDTH][MAP_HEIGHT] is empty .Failed to assign value to data member x ,y ,index . I don't know where is the problem .

Answer 1

Point* mapPoints[MAP_WIDTH][MAP_HEIGHT];
...
 mapPoints[px][py]->x = px;

mapPoints is an array of pointers. The pointers does not point anywhere. As the mapPoints is a global variable, it is initialized with nullptr . nullptr is invalid pointer, dereferencing it mapPoints[px][py]->x is undefined behavior and most probably causes something similar to "segmentation fault" or other strange fault.

You need to have the memory, not pointers, if you want to store something.

Point mapPoints[MAP_WIDTH][MAP_HEIGHT];
...
 mapPoints[px][py].x = px;

Also in this line:

Point* points = new Point[RANDOM_POINTS];

you are allocating RANDOM_POINTS points. But you never delete them. This leaks memory. Just do:

Point points[RANDOM_POINTS];