我无法在列表 c++ 中为 object 赋值

Question

I have struct Point, it contains setters for x and y:

struct Point {
    int x;
    int y;

    Point() {
        x = 0;
        y = 0;
    }

    Point(int _x, int _y) {
        x = _x;
        y = _y;
    }

    Point& operator=(Point other) {
        swap(x, other.x);
        swap(y, other.y);
        return *this;
    }

    void setX(int _) { x = _; }

    void setY(int _) { y = _; }

};

Also I have Element for bidirectional list, its data is Point:

struct Element {
    Point data;
    Element *left;
    Element *right;

    Element(Point data) {
        this->data = data;
        left = right = this;
    }

};

And also I created List structure, Point get(int i) returns Point I want to change:

struct List {
    Element *first;
    Element *last;
    int length;

    List() {
        first = last = nullptr;
        length = 0;
    }

    Point add(Point data) {
        Element *e = new Element(data);
        length++;
        if (!last) {
            first = last = e;
            return data;
        }
        e->left = last;
        last = last->right = e;
        return data;
    }

    Point get(int i) {
        if (i >= length && i<0) return Point();
        Element *t = first;
        while(i--) t = t->right;
        return t->data;
    }

};

Then in main I create List, I add Point to this list and then I try to change x and y values, but they don't change:

int main() {
    ....
    List snake;
    snake.add(Point(3,4));
    cout << snake.get(0).x << " " << snake.get(0).y << endl;
    snake.get(0).setX(99);
    snake.get(0).setY(99);
    cout << snake.get(0).x << " " << snake.get(0).y << endl;
    .....
}

Please, help? Why don't x and y values change and how to fix it?

Answer 1

This method:

Point get(int i) {
    if (i >= length && i<0) return Point();
    Element *t = first;
    while(i--) t = t->right;
    return t->data;
}

returns a Point by value. The object returned is not the object in the list. Hence here:

snake.get(0).setX(99);

You do setX(99) of a Point , but that Point is the temporary object returned from get . You probably want to return a reference instead. However, then you need to find a different solution for if (i >= length && i<0) return Point(); , ie the case when the index is out of bounds, because you cannot return a reference to a local variable. You can either throw an exception or simply assume that the passed index is a valid one. Change it to:

Point& get(int i) {
   //^  return reference
    //if (i >= length && i<0) return Point();
  //^^ DO NOT return a reference to  ^^ local object !!!
    Element *t = first;
    while(i--) t = t->right;
    return t->data;
}