Having trouble with pointers in classes - c++

Question

I don't have any trouble when I declare
SDL_Surface *dot = NULL;
globally, but if the SDL_Surface is unique to the class I cant set it to NULL, so thought it would be fine if I declare it in the constructor like
dot = load_image( "dot.bmp" );
but I still get a
Unhandled exception at 0x1002b195 in Uber Mario.exe: 0xC0000005: Access violation reading location 0x0000013c.
on the load_image which returns a SDL_Surface*, sometimes that happened to be because the image was bad or a certain img filetype so i tried another image that does work elsewhere but it still errors like this.
i think im just not using the pointers correctly, even though i studied pointers in school and have read facts on them, for some reason i always have trouble with them. load_image returns a *SDL_Surface so i need to use a pointer...i think.
here is the class:
class Character
{
private:
int yVel, xVel;
int xAcc, yAcc;
int spd, maxV;
int JumpPower;
int FacingRight, FacingLeft;//directing status 0 or 1
bool Flying, onGround;
//Type of particle
SDL_Surface *type;
public:
Shine *myShine;
Animation *walking;
SDL_Surface *dot;
//Offsets
SDL_Rect Rect;
Character();
void handle_input();
void move();
void show();
void togglefly();
void jump();
void whereami();// check and set various characters statuses
};
Character::Character()
{
//Set offsets
Rect.x = 150;
Rect.y = 150;
Rect.w = 20;
Rect.h = 20;
yVel = 0;
xVel = 0;
yAcc = 0;
xAcc = 0;
maxV = 30;
spd = 2;
JumpPower = 40;
Flying = true;
myShine = new Shine(Rect.x, Rect.y);
// walking = new Animation("mario.bmp", 3, 0, 0, Rect.w, Rect.h);
dot = new SDL_Surface();
dot = load_image( "dot.bmp" );
myShine->setpos(Rect);
myShine->setRange(Rect.h*1.5);
}
the load image function:
SDL_Surface *load_image( std::string filename )
{
//The image that's loaded
SDL_Surface* loadedImage = NULL;
//The optimized surface that will be used
SDL_Surface* optimizedImage = NULL;
//Load the image
loadedImage = IMG_Load( filename.c_str() );
//If the image loaded
if( loadedImage != NULL )
{
//Create an optimized surface
optimizedImage = SDL_DisplayFormat( loadedImage ); //EXCEPTION OCCURES HERE
//Free the old surface
SDL_FreeSurface( loadedImage );
//If the surface was optimized
if( optimizedImage != NULL )
{
//Color key surface
SDL_SetColorKey( optimizedImage, SDL_SRCCOLORKEY, SDL_MapRGB( optimizedImage->format, 0, 0xFF, 0xFF ) );
}
}
//Return the optimized surface
return optimizedImage;
init function
bool init()
{
//Initialize all SDL subsystems
if( SDL_Init( SDL_INIT_EVERYTHING ) == -1 )
{
return false;
}
//Set up the screen
screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE );
//If there was an error in setting up the screen
if( screen == NULL )
{
return false;
}
//Set the window caption
SDL_WM_SetCaption( "Particle Test", NULL );
//Seed random
srand( SDL_GetTicks() );
//If everything initialized fine
return true;
}

Answer 1

By the way, aren't you leaking resources executing that code:
dot = new SDL_Surface();
dot = load_image( "dot.bmp" );
Doing load_image causes you to loose pointer to SDL_Surface() object hence you cannot delete it later.
Answer to your main issue
Call SDL_Init before using SDL_DisplayFormay and it should work. Citation from SDL documentation.
Newbie hint
You have to call SDL_Init before using the SDL_DisplayFormat function. If you don't, your program will crash with an access violation.

Answer 2

You know, dot is a pointer,and it was allocated dynamiclly.
SDL_Surface *dot=new SDL_Surface();means that you allocate a storage space in heap,and dot index this space.
And,the function load_image(string) return a SDL_Surface type value,that is an object，the return object was assignmented to the pointer dot,then pointer dot changed it's direction,then this makes a memory leak.
You can modify the program by this:
SDL_Surface *dot;
dot = load_image("dot.bmp");
thank you.