Class field resets to zero

Question

im trying to make a simple game engine, it consists of 2 classes, Game

class Game
{
    protected:
        SDL_Event event;
        vector<Sprite> render_queue;

        void render();

    public:
        int SCREEN_WIDTH,
            SCREEN_HEIGHT,
            SCREEN_BPP,
            FPS;
        SDL_Surface *screen;

        Game(int width, int height, int fps);
        void reset_screen();
        void set_caption(string caption);
        bool update();
        void quit();
        void add_sprite(Sprite spt);
};

and Sprite

class Sprite
{
    public:
        float x,y;
        SDL_Surface* graphics;

        Sprite();
        void draw(SDL_Surface *target);
        void loadGraphics(string path);
};

when im trying to change x or y of sprite it resets to 0 on next frame!

main.cpp

int main(int argc, char* args[])
{
    Game testing(640, 480, 50);
    testing.set_caption("Test");

    Sprite s1;
    Sprite s2;

    s1.loadGraphics("img1.png");
    s2.loadGraphics("img1.jpg");

    testing.add_sprite(s1);
    testing.add_sprite(s2);

    while(!testing.update())
    {
        s1.x = 100;
        s2.y = 200;
    }

    return 0;
}

Answer 1

You make copies of the sprites when adding them to the Game:

class Game
{
        /* ... */
        void add_sprite(Sprite spt);
};

That's why the lines s1.x = 100; and s2.y = 200; (operating on a different copy in the main()) have no effect.

I think the method should be instead defined like that:

class Game
{
        /* ... */
        void add_sprite(Sprite &spt);
};

That way Game would be using the Sprite objects from the main().

Answer 2

testing.add_sprite(s2); makes a copy of sprite, so changing the value of s1,s2 in the main code has no effect on the copy in testing.

You need to pass by reference or pointer to to add_sprite()