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A little help with classes and SDL 2.0

3 posts in this topic

Hello, everyone. This is my first post, so I'm sorry if I've missed something.


I've made 2 classes - a Paddle and a Ball class. Both within them have a SDL_Rect object which I need to get out for the SDL_HasIntersection() function, but, no matter what I try, I'm getting this error:


\dir\ - timer.cpp|140|error: cannot convert 'SDL_Rect' to 'const SDL_Rect*' for argument '1' to 'SDL_bool SDL_HasIntersection(const SDL_Rect*, const SDL_Rect*)'|


\dir\ - timer.cpp|141|error: cannot convert 'Ball' to 'const SDL_Rect*' for argument '1' to 'SDL_bool SDL_HasIntersection(const SDL_Rect*, const SDL_Rect*)'|


Both of the classes have a Draw(SDL_Renderer* gRenderer) method which I think acts just like SDL_FillRect(), so I tried making it available like this:


void Paddle::Draw(SDL_Renderer* gRenderer){
    SDL_Rect Paddle;
    Paddle.x = posX;
    Paddle.y = posY;
    Paddle.w = PADDLE_WIDTH;
    Paddle.h = PADDLE_HEIGTH;

    SDL_SetRenderDrawColor(gRenderer, 255, 255, 255, 255);
    SDL_RenderDrawRect(gRenderer, &Paddle);
    thisPaddle = Paddle;


AND in my game loop - SDL_HasIntersection(ball.thisBall, paddle2.thisPaddle), but it didn't work. Using a method like this


SDL_Rect Ball::getRect()
    return thisBall;


also didn't work. I'm guessing getting rid of the classes isn't a great idea?

Any help, tips, code is appreciated.


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You seem to be having basic C/C++ issues; nothing about this is really specific to SDL.


You can't convert SDL_Rect to a const SDL_Rect* becaue the former is a _value_ and the later is a _pointer_.  Use & to get the address of a value (get a pointer pointing to it).

// this is a value (created locally)
SDL_Rect rect;

// this is a pointer that points to rect
SDL_Rect* pRect = ▭

// this is illegal and will not compile since a value doesn't not automtically convert to a pointer
SDL_Rect* pRect = rect;

// this is legal and compiles because a value does automatically convert to a C++ reference
SDL_Rect& rRect = rect;

You can't convert a Ball to an SDL_Rect because a Ball is not a rectangle.  You need to access the member variable that is the rect (thisBall, I guess; your choice of names is very unconventional and not the easiest to follow).  You can't take the address of a temporary, so you can't return an SDL_Rect from a method and then take the address of it, e.g. this is illegal:

// illegal if getRect returns a value, since the result is an rvalue (temporary)

You should just directly return a pointer to the member variable from your getRect method or you should return a C++ reference (which you can take the address of):

// do this for ball and paddle
const SDL_Rect& Ball::getRect() const { return thisBall; }

// this is how you call the code
SDL_HasIntersection(&paddle.getRect(), &ball.getRect());

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Sean, you can't imagine how grateful I am. Thank you very much! I was waiting for someone to reply.


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