This second post on vtable will be consecrated to the mechanics of virtual inheritance. It will get slightly more complex, so bear with me.

Disclaimer

Like it was said in part 1, all of this is entirely implementation dependent, C++ has no concept of vtable. The C++ specification only provides specific behaviors that must be guaranteed and this is simply how most compilers implement it. . Using them manually is absolutely not portable. The only purpose of this code is to have fun learn and understand. Don’t use it for any other purpose

Code tested on

  • GCC 5.3 - Ubuntu 14.04

Simple Inheritance

First, let’s see a simple case where virtual inheritance is needed. We have four classes, GuiElement, Label, Clickable and Button. They form an hierarchy where both Label and Clickable inherited from GuiElement. Button needs to be drawn so it inhertie from Label. It also need to be clickable so of course it also inherits from Clickable. One code worth ten thousand words right?

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struct GuiElement
{
    int id;
};

struct Label : public GuiElement
{
    int a;
};

struct Clickable : public GuiElement
{
    int b;
};

struct Button : public Label, public Clickable
{
    int c;
};

And to make it even easier to visualize, UML diagram:

UML_vtable_simple_inheritance

That’s not quite what we want. If we had an object button of type Button we could see that it has two member variable id, accessible by button.Label::id and button.Clickable::id. We’ll still try to understand how it work for simple inheritance. Here we can look at how our structs are organized in memory.


Label Clickable
GuiElement::id GuiElement::id
Label::a Clickable::b


The magic of this configuration is that you can very easily cast between Label* and GuiElement*. Just like in all the code and explanation we’ll see, Label and Clickable are equivalent. Minus the name change, the same code and concept applies to both in the exact same way. Unfortunately, it is not that simple for Button.


Button
Label::GuiElement::id
Label::a
Clickable::GuiElement::id
Clickable::b
Button::c


As you can see, we can cast with ease between Button* and Lable*.

To cast from Button* to Clickable* we need to adjust our pointer to point to the “Clickable” part of Button. That can be done by adding an offset to our pointer so that we are pointing to Clickable::GuiElement::id.

  Button
  Label::GuiElement::id
  Label::a
-> Clickable::GuiElement::id
  Clickable::b
  Button::c


And now we are pointing to a valid Clickable object. To cast it back to Button* we just have to remove that offset from the pointer so that we point to Label::GuiElement::id again.

You might want to note that you can’t do GuiElement* guiElement = button. Because button has two instance of GuiElement, the compiler wouldn’t know to which you want to refer . If I’m begin stubborn and decide to try anyway, GCC gently remind me that I’m an idiot:

error: ‘GuiElement’ is an ambiguous base of ‘Button’

It is still possible to do it by first casting to Label* or Clickable*:

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GuiElement* guiElementLabel = (Label*)button;
GuiElement* guiElementClickable = (Clickable*)button;

And that’s pretty much it for basic inheritance, let’s dive in the core subject.

Virtual Inheritance

So we’ve seen that in our example that, when using simple inheritance, we don’t get exactly what we wanted, we got two instances of GuiElement for each Button. That’s an issue we can fix with one magic word!

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struct Label : virtual public GuiElement
{
    int a;
};

struct Clickable : virtual public GuiElement
{
    int b;
};

By making Label and Clickable virtually inherit from GuiElement we get:

UML_vtable_simple_inheritance

Much better! Now our Buttons only have one instance of GuiElement. This may look easier from a programmer stand point, but it makes the life harder for the compiler. And since you’re here, for you too.


Button Label Clickable
vptr_Label vptr vptr
Label::a Label::a Clickable::b
vptr_Clickable GuiElement::id GuiElement::id
Clickable::b    
Button::c    
GuiElement::id    


Unexpected right? At least it was for me at first. Let’s see how it works! Note that the following code is not the actual class declaration, rather the result of compilation. it’s what the object might will look like in memory. To avoid confusion, I’ll add the sufix _real at the end of structs name. And don’t forget that this is only to give you an idea of how it work, details might change depending of the compiler.

First case, Label:

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struct Label_vtable
{
    // offset are in byte
    int offsetToVariable_GuiElement_id; //offset from the vptr to the variable
    int offsetToTop; //offset from the vptr to the top of the class
    type_info* typeInfoPtr; //pointer to the type_info for this object
};

struct Label_real
{
    Label_vtable vptr; //pointer to the vtable
    int label_a; // member varible
    int guiElement_id; // member variable inherited from GuiElement
};

Note the arriving of a new friend, typeInfoPtr. It is used for various RTTI operations, eg: dynamic_cast, typeid.

When we want to access the member variable id, here represented by guiElement_id. We have to look into the vtable to see at what offset it is in the class. Assuming we have a Label* named label it would pretty much like this.

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Label_vtable* vtable = label.vptr;
int8_t offset = vtable->OffsetToVariable_GuiElement_id; // the offset is in byte
int8_t* varaibleAddress = (int8_t*)label + offset;
id = *(int*)varaibleAddress;

or for those that prefer really dense code.

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id = *(int*)((int8_t*)label + label.vptr->OffsetToVariable_GuiElement_id);

But we didn’t initialize our label with valid offset yet! Adding a constructor to do so, we would end up with this. Note that we get a memory leak here since the vtable get allocated but never is freed.

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Label_real::Label_real()
{
    // create the vtable
    vptr = new Label_vtable();
 
    // calcualte the offset from the vptr (wich is as the top of the class) to the variable
    // label + offset == &label->variable
    vptr->OffsetToVariable_GuiElement_id = sizeof(Label_vtable*) + sizeof(int);
 
    // the vptr is at the top of the class
    // so the offset from the vptr to the
    // top of the class is obviously 0
    vptr->OffsetToTop = 0;
 
    // theoricaly that's how we should do it but the copy constructor of type_info is private
    // anyway we dont actually need it for our exemple
    // I'll still leave it here as reference
    //vptr->typeInfoPtr = new type_info(typeid(Label));
}

The same applies for Clickable.

I hope that you’re still following and that it’s not too messy. If everything is confusing and you’re lost then it’s my fault, I apologize.

Now we need to understand how it works for Button, especially for casting to Label* or Clickable*.

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struct Button_vtable
{
    Label_vtable vtable_label;
    Clickable_vtable vtable_clickable;
};

struct Button_real
{
    Label_vtable* vptr_lable; //pointer to the lable vtable
    int Label_a; // member varible inherited from Label

    Clickable_vtable* vptr_clickable; //pointer to the clickable vtable
    int Clickable_b; // member varible inherited from Clicackable

    int Button_c; // member varible

    int GuiElement_id = 10; // member variable inherited from GuiElement
};

Do you see it? If you don’t, no worries! Here’s how it goes. We defined Label_real like this.

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struct Label_real
{
    Label_vtable vptr; //pointer to the vtable
    int label_a; // member varible
    int guiElement_id; // member variable inherited from GuiElement
};

The first two members fit perfectly in the layout of Button. For the third one, remember, it’s position is calculated from an offset located in the vtable. So we just have to adjust this offset so that it point to our guiElement_id in Button. To make it more clear(yes I’m better with code than words) here’s our beloved constructor.

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Button_real::Button_real()
{
    vptr_label = new Label_vtable();

    // the variable is separeted from the vptr by three int and two pointer
    vptr_label->OffsetToVariable_GuiElement_id = sizeof(void*)*2 + sizeof(int)*3;

    // this vptr is still at the top
    vptr_label->OffsetToTop = 0;

    vptr_clickable = new Clickable_vtable();

    // the variable is separated from the vptr by two int and one pointer
    vptr_clickable->OffsetToVariable_GuiElement_id = sizeof(void*) + sizeof(int)*2;

    // this vptr is separated from the top by one int and one pointer
    vptr_clickable->OffsetToTop = sizeof(void*) + sizeof(int);
    
    // note that the typeInfoPtr of both vtable would be pointing to
    // the type_info of Button 
}

Now you know how it works under the hood when casting from derived class to base class (upcasting), even when virtual inheritance is involved. You can brag to your friend about it and look cool. Until they ask you about downcast… You might be tempted to say that’s it as simple as it is for non-virtual inheritance, you just have to substract the offset you added previously right? Well, no, that would be too easy wouldn’t it be?

To understand why, we need yet another class.

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struct Slider : public Label, public Clickable
{
    int d;
    int e;
};

And the obligatory UML diagram.

UML_vtable_simple_inheritance

Consider the following case. We have want to cast from GuiElement* to Label*, don’t forget it could be pointing to any of the following; GuiElement, Label, Clickable, Button or Slider. Now the problem is that GuiElement by itself carry no type information.

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struct GuiElement_real
{
    int id;
};

That means it could be any of the following case That means that the GuiElement could be at any of those point and we have no way to know which one it is. Since we dont have RTTI we can’t know how to cast from GuiElement* to Label* because we dont know where we point relatively to the Label, and in some case there is no Label.

  Slider   Button   Label   Clickable   GuiElement
  vptr_Label   vptr_Label   vptr   vptr      -> GuiElement::id
  Label::a   Label::a   Label::a   Clickable::b    
  vptr_Clickable   vptr_Clickable      -> GuiElement::id      -> GuiElement::id    
  Clickable::b   Clickable::b            
  Slider::d   Button::c            
  Slider::e      -> GuiElement::id            
-> GuiElement::id                

You can actually try it and the compiler will tell you.

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Label* label = dynamic_cast<Label*>(guiElement);
error: cannot dynamic_cast ‘guiElement’ (of type ‘struct GuiElement*’) to type ‘struct Label*’ (source type is not polymorphic)

As the compiler pointed out, the problem is that GuiElement is not polymorphic. The best way to fix this is to add a virtual destructor to GuiElement. Anyway, every class which gets inherited from should have a virtual destructor to ensure proper destruction of every class.

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struct GuiElement
{
    virtual ~GuiElement() = default;
    int id;
};

Now the compiler knows that GuiElement needs a vtable and will include one. And since the vtable include RTTI (via typeInfoPtr) we will be able to perform our cast! But first, since we modified GuiElement we need to redefine Button_real and Button_vtable.

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struct Button_vtable
{
    Label_vtable vtable_label;
    Clickable_vtable vtable_clickable;
    GuiElement_vtable vtable_guiElement;
    
    //here would also be a function pointer to the virtual destructor 
    //~GuiElement()
};

struct Button_real
{
    Label_vtable* vptr_lable; //pointer to the lable vtable
    int Label_a; // member varible inherited from Label

    Clickable_vtable* vptr_clickable; //pointer to the clickable vtable
    int Clickable_b; // member varible inherited from Clicackable

    int Button_c; // member varible

    GuiElement_vtable* vptr_guiElement; //pointer to the guiElement vtable
    int GuiElement_id = 10; // member variable inherited from GuiElement
};

From all the information here you should be able to work out what GuiElement_vtable looks like. Of course vptr_guiElement need to be added to Lable_real and Clickable_real. And every constructor need to be updated so that they give the correct offset.

One last point, you might wonder what’s offsetToTop doing in vtables. It’s used when casting to void* so that you can simply substract the pointer by the offset to get the base of the most derived class.



And that’s pretty much it. I hope it was clear enough. If you have any suggestion or question, feel free to comment in the box below or to contact me directly.