在函数中使用继承的类？

Question

Answered: Thanks to @AndyProwl pointing out some stuff to me, I found out the problem was in my ScreenImage.Update() function. I had flopped back and forth on writing implementation for that function (as the ScreenImage doesn't need to Update, it just needs to be drawn) and somehow accidentally left the Update function on my interface when I erased the implementation...

LET THAT BE A WARNING TO YOU KIDS. ~The more you know~

Original Question:

I'm making a game, and I have a Screen object that keeps track of items on a given screen (ie images, text, menu options, etc.). My goal is to pass these items to the Screen in the form of an abstract ScreenElements object that could, in turn, be defined as more concrete objects, like a ScreenImage or ScreenText etc.

I went through a headache trying to get the vector (basically treating it as a stack for ScreenElements) to play nice with the different inherited classes by making it a vector of pointers to the base object, but then that means I have to declare objects inheriting from ScreenElements outside of a function so the pointer doesn't become invalid when the declaration goes out of scope. So then I made a function in my Screen object to allow me to "AddElement" and I thought the same pointer method would work... it apparently doesn't.

Here's my source code just to clear things up. This is Screen.h.

#ifndef SCREEN_H
#define SCREEN_H

#include <vector>

#include "ScreenElement.h"

class Screen
{
protected:
    bool enabled;

    std::vector<ScreenElement*> screen_elements;
public:
    //Gameloop Functions
    void Init();
    void Update(int ticks); //Enabled will likely determine if this is called
    void Draw(); //And this perhaps on a visible bool
    void Destroy();

    //Concept functions
    //void TransitionIn(data);
    //void TransitionOut(data);

    void AddElement(ScreenElement *Element); //function in question

    //Allow external use of enabled feature
    void ToggleEnable(bool Enabled) { enabled = Enabled; }
    bool IsEnabled() { return enabled; }
};

#endif // SCREEN_H

And my Screen.cpp file

#include "Screen.h"

void Screen::Init()
{
    enabled = true;
}

void Screen::Update(int ticks)
{
    for(int i = 0; i < screen_elements.size(); i++)
        screen_elements[i]->Update(ticks);
}

void Screen::Draw()
{
    for(int i = 0; i < screen_elements.size(); i++)
        screen_elements[i]->Draw();
}

void Screen::Destroy()
{
    screen_elements.clear();
}

void Screen::AddElement(ScreenElement *Element)
{
    screen_elements.push_back(Element);
}

I need my AddElement to work with any inherited form of ScreenElement, such as ScreenImage for instance, and store it into the vector.

EDIT: I guess I should also put up some source code from my ScreenElement class too. When I attempted to add a virtual destructor it only caused another error in addition to the first which seems to be related to the inherited class's constructor (In function ScreenImage::ScreenImage() undefined reference to 'vtable for ScreenImage'). So here's my ScreenElement.h

#ifndef SCREENELEMENT_H
#define SCREENELEMENT_H

#include "Globals.h"

class ScreenElement
{
protected:
    s_Position screen_position;

    bool enabled;
public:
    virtual void Update(int ticks) {}
    virtual void Draw() {}
    virtual void Destroy() {}

    void ToggleEnable(bool Enabled) { enabled = Enabled; }
    bool IsEnabled() { return enabled; }
};

#endif // SCREENELEMENT_H 

Which in turn was used to create ScreenImage.h

#ifndef SCREENIMAGE_H
#define SCREENIMAGE_H

#include "Globals.h"
#include "ScreenElement.h"
#include "SpriteSheet.h"

class ScreenImage: public ScreenElement
{
private:
    SpriteSheet *sprite_sheet;

    int sprite_sheet_frame;
public:
    void Init(SpriteSheet *Sprite_Sheet, float X, float Y, int SpriteSheetFrame = 0);
    void Update(int ticks);
    void Draw();
    void Destroy();
};

#endif // SCREENIMAGE_H

And has its own implementations for base functions in ScreenImage.cpp.

Answer 1

After initializing an instance of Screen called SplashScreen and a ScreenImage called SplashScreen_Image, I use "SplashScreen.AddElement(&ScreenImage);" Then it throws me an error about having 'undefined reference to "vtable for ScreenImage"

Judging from the error you are reporting, the problem seems to be that you have provided a declaration, but not a definition , for some virtual function of your class.

For instance, trying to compile the following code will produce a very similar error:

struct X
{
    virtual void foo() { }
};

struct Y : X
{
    virtual void foo(); // Missing definition...
};

int main()
{
    Y y;
    y.foo(); // The compiler will complain about undefined reference
             // to vtable for Y...
}

Here is the corresponding live example .