help with pointers?

[N8RzwA7f]

hello,

I have a gameBalls class and a gameWindow class.

so in gameWindow.h I have : gameBalls* balls;
in gameWindow.cpp I have to include a gameBalls object so I pass it in the constructor (as a pointer to gameBalls).
as in: gameWindow(gameBalls* balls) : balls(balls) {}

in gameBalls constructor I initialze a pointer to surface
gameBalls::gameBalls() {
pBalls = new SDL_Surface(); // SDL_Surface* pBalls in gameBalls.h

then in main I create a gameWindow object (pass it a gameBalls object pointer)
gameWindow window = new gameWindow(&balls);
and try to access this balls pointer to pBalls in gameWindow.cpp
as so : balls->pBalls .... this creates an error ?


any ideas why ?
hope somebody can help thanks so much :)

[Peter87]

oOoOo wrote:

balls->pBalls .... this creates an error

You can't access pBalls from outside the class if it's a private member.

oOoOo wrote:

pBalls = new SDL_Surface();

SDL is a C library so it doesn't have constructors and destructors. Instead you need to use the functions provided by the SDL library to construct and destruct the objects.

To create a SDL_Surface you would have to use a function such as SDL_CreateRGBSurface or SDL_LoadBMP.
https://wiki.libsdl.org/SDL_CreateRGBSurface
https://wiki.libsdl.org/SDL_LoadBMP

To destroy a SDL_Surface you should use SDL_FreeSurface.
https://wiki.libsdl.org/SDL_FreeSurface

[N8RzwA7f]

pBalls is public does that make a difference?

[Peter87]

It just rules out one of many possible explanations. What does the error message say?

[N8RzwA7f]

I removed the pBalls = new SDL_Surface(); statement from the gameBalls constructor but it still
crashes at: balls->pBalls = IMG_Load("ball1.png"); //in gameWindow.cpp

hope you can help :)

[Peter87]

Does balls point to a valid gameBalls object?

[N8RzwA7f]

I think so . I have in main:

gameBalls* balls1;
gameWindow* window = new gameWindow(balls1);


in gameWindow.h

private: gameBalls* balls1;

ing gameWindow.cpp:

gameWindow(gameBalls* balls1) balls1:balls1 {}
gameWindow::somefunction() { balls1->pBall= IMG_Load("ball1.png");

in gameBalls h:
public: SDL_Surface* pBall = NULL;



thanks! :)

[Peter87]

You have just created a pointer but you have not set it to point anywhere.

You can create the gameBalls object with new as you have done with your other objects.

gameBalls* balls1 = new gameBalls;
gameWindow* window = new gameWindow(balls1);

Pointers are error prone and often make the code more complicated than it has to be so you might want to consider not using them when you don't have to.

gameBalls balls1;
gameWindow window(balls1);

[N8RzwA7f]

I would , but in order to add the gameBalls object to gameWindow it needs to be a pointer? I think?
I tried adding it as a normal object but I got errors about it .
I changed my code with added new , but it still crashes (for other reasons). I doubt you will want to take a look?

thanks.

[Peter87]

oOoOo wrote:

in order to add the gameBalls object to gameWindow it needs to be a pointer? I think?

You can always get a pointer to an object using the address-of operator &.

gameBalls ballsObject;
gameBalls* ballsPointer = &ballsObject;

In some situations you can use references as a substitute for pointers.

oOoOo wrote:

I changed my code with added new , but it still crashes (for other reasons). I doubt you will want to take a look?

I'm going to bed now. If you post the code someone else might be able to help, or I can have a look in the morning.