c++结构和方法，未正确返回

Question

我有一个子程序 Map build(Map the_map) 被调用来更新我的变量 new_map 并且 map 正在子程序内部更新但是当它返回时它返回相同的 map 未更新，
笔记:
我在方法中使用 cout 语句进行了测试，并且在方法返回到开关之后...

这是我的代码片段:
1 结构细节
2 和调用子程序的时刻
3、子程序代码

结构

struct MapItem {    
    char type = 'E';    
};

struct Map {
    int size = 0;
    MapItem *items;
};

在 switch 中调用子程序(如果用户选择构建选项)在 map 上构建

case BUILD:
                new_map = build(new_map);
                break;

子程序

Map build (Map the_map) {

    char building_code;
    int coordinate_x;
    int coordinate_y;
    int build_location;

    cout << "Enter x and y coordinate: ";
    cin  >> coordinate_x;
    cin  >> coordinate_y;

    build_location = (coordinate_x+(coordinate_y*the_map.size));

    cout << "Enter a building code: ";
    cin >> building_code;

    the_map.items[build_location].type = building_code;

    return the_map;
}

Answer 1

尝试通过引用传递 map 。

像这样:

void build (Map& the_map) {

    char building_code;
    int coordinate_x;
    int coordinate_y;
    int build_location;

    cout << "Enter x and y coordinate: ";
    cin  >> coordinate_x;
    cin  >> coordinate_y;

    build_location = (coordinate_x+(coordinate_y*the_map.size));

    cout << "Enter a building code: ";
    cin >> building_code;

    the_map.items[build_location].type = building_code;
}

和

case BUILD:
                build(new_map);
                break;

当您通过引用传递时，您可以直接更改变量(即映射)并避免所有复制的复杂性。

顺便说一句:我假设您已经在代码中的其他地方为 MapItems 保留了内存，但您可能需要在构建函数内部进行一些范围检查，这样您就不会在分配的内存之外进行写入。

如果您已正确初始化，您的初始代码应该没问题。我这样试过:

    Map build(Map the_map) {

    char building_code;
    int coordinate_x;
    int coordinate_y;
    int build_location;

    cout << "Enter x and y coordinate: ";
    cin >> coordinate_x;
    cin >> coordinate_y;

    build_location = (coordinate_x + (coordinate_y*the_map.size));

    // Range check
    if ((build_location >= (the_map.size*the_map.size)) || (build_location < 0))
    {
        cout << "Invalid" << endl;
        return the_map;
    }

    cout << "Enter a building code: ";
    cin >> building_code;

    the_map.items[build_location].type = building_code;

    return the_map;
}


int main()
{
    // Just a const so that map size can be change here
    const int mapSize = 3;

    // Create the map
    Map new_map;

    // Initialize map size
    new_map.size = mapSize;

    // Allocate memory for the MapItems
    new_map.items = new MapItem[mapSize * mapSize];

    // Do some simple test...
    for (int i = 0; i < mapSize * mapSize; i++) cout << new_map.items[i].type << " ";
    cout << endl;

    for (int j = 0; j < 3; j++)
    {
        new_map = build(new_map);
        for (int i = 0; i < mapSize * mapSize; i++) cout << new_map.items[i].type << " ";
        cout << endl;
    }

    // Deallocate memory
    delete[] new_map.items;

    return 0;
}


E E E E E E E E E
Enter x and y coordinate: 2 0
Enter a building code: Y
E E Y E E E E E E
Enter x and y coordinate: 0 2
Enter a building code: Z
E E Y E E E Z E E
Enter x and y coordinate: 2 2
Enter a building code: P
E E Y E E E Z E P

但我会推荐传递引用。