C++ Empty Pointer

Question

So I am new to C++ and I am trying to use an inner class like this:

struct PreviousButton {
    SelectionScreen* ss;

    PreviousButton(SelectionScreen* sss) {
        ss = sss;
    }

    void ClickAction() {
        //images is a vector with images. in here it is empty
        ss->images;
    }
};

This inner class is inside the class SelectionScreen and I need to perform a click action and need some of the variables of the selectionscreen.

SelectionScreen:

class SelectionScreen {
public:
    void AddImage(Image img);
    std::vector<Image> images;

How I fill the vector:

Image* image = new Image{ };
AddImage(*image2);

AddImage method:

void SelectionScreen::AddImage(Image img)
{
    images.push_back(img);
}

But when I try to use the selectionscreen inside the class all of the variables are empty. But when I look into selectionscreen all of the variables are set.

The way I pass the SelectionScreen:

PreviousButton* previousButton = new PreviousButton(*ren, this);

How SelectionScreen gets initialized: (this method is called from the main)

int Program::Render() {
    bool quit = false;
    MenuScreen m = SelectionScreen{ Sdl_Renderer };
    // change current screen to selectionscreen
    ScreenController::GetInstance().ChangeMenu(m);

    while (!quit) {
    // handle user input and repaint
    }

    // delete all windows if quit is true
    SDL_DestroyRenderer(Sdl_Renderer);
    SDL_DestroyWindow(Sdl_Window);
    SDL_Quit();
    return 0;
}

Does anyone knows why my variables are empty?

Answer 1

This isn't addressing the immediate issue you describe, but you have a serious problem here:

  ss->currentImageIndex = ss->images.capacity() - 1;

images.capacity() - 1 is not guaranteed to return a valid index into your vector. You should use images.size() - 1.

std::vector::capacity() tells you the storage space currently allocated for the vector, not the number of elements it actually contains.

UPDATE

When you assign a SelectionScreen to a MenuScreen like this

MenuScreen m = SelectionScreen{ Sdl_Renderer }; 

you "slice" the SelectionScreen object (see What is object slicing?).

This line creates a new MenuScreen object, m, by copy-constructing it from a temporary SelectionScreen object. The new MenuScreen object, m, is not a SelectionScreen. It is also a completely distinct object from the temporary SelectionScreen created on the right-hand side of this expression.

Your PreviousButton, which I assume is created in the constructor of SelectionScreen, holds a pointer to the temporary SelectionScreen, which the compiler is free to delete once this line has completed.

To fix this, you could use this initialisation instead:

MenuScreen& m = SelectionScreen{ Sdl_Renderer };

In this case, m is a reference to your (temporary) SelectionScreen - this is good because (a) references are polymorphic, so it still knows that it's really a SelectionScreen, not just a MenuScreen and (b) it's a reference to the exact same SelectionScreen object created on the right-hand side, which your PreviousButton has a pointer to. I've also bracketed the "temporary" bit now, because taking a local reference to a temporary guarantees it will exist for as long as the reference does (see e.g. Does a const reference prolong the life of a temporary?).

Note that I am assuming here that you only make use of this SelectionScreen within the while (!quit) loop that follows, i.e. the screen can safely be deleted at the end of this int Program::Render() method.