Custom copy assignment operator crashes the program (C++)

Question

I'm creating a custom class for automaticly cleaning up memory. The class contains a pointer to an SDL_Surface object and calls it's refcount when needed. The problem is when I implement the custom copy assigment operator the system crashes with the following code:

"Unhandled exception at 0x771a15de in xyz.exe: 0xC0000005: Access violation reading location 0xcccccd04."

and the object attribute "address" suddenly gets the value "0x0000ffff ", whilst using the default copy assignment operator it runs perfectly fine.

Answer 1

You're using the refcount incorrectly. SDL_FreeSurface will decrement the refcount, and when called on a null pointer, is a no-op. So, your assignment operator should look like this:

const Image& Image::operator=(const Image& other){
    if (img != other.img){
        SDL_FreeSurface(img);
        img = other.img;
        if (img != NULL)
            img->refcount++;
    }
    return *this;
}

And your destructor should look like this:

Image::~Image(){
    SDL_FreeSurface(img);
}

Also, in your default constructor, you should initialize img to a null pointer.

Image::Image() :img(NULL) {}

See the second to last example on this page: http://sdl.beuc.net/sdl.wiki/SDL_Surface