Base Class Overloaded Virtual Function

leeor_net · 07-12-2009, 02:09 PM

Hello again.

I've been working with a somewhat complex class heirarchy and have run into a problem. More than likely it stems from not fully understanding how C++ derived classes inherit from base classes.

To keep things simple, I have the following three classes defined as such:

Code:

class Foo
{
    private:
        virtual void myFunc() { printf("Hello!"); }
        virtual void myFunc2() { printf("'ello!"); }
};

class Bar : public Foo
{
    private:
        void myFunc() { printf("Hello 2!"); }
        void myFunc2() { printf("'ello 2!"); }
};

class FooBar : public Bar
{
    private:
        void myFunc() { printf("Hello 3!"); }
        void myFunc2() { printf("'ello 3!"); }
};

The weird thing that's happening is that when I call the overloaded function from FooBar myFunc() from a list of Foo's (e.g., std::vector<Foo*>), the correct function is called. But myFunc2() calls the function from Foo, not from Bar or FooBar.

I'm not sure this is clear enough but I'm hoping I provided enough information and that my problem is something really simple or that I just overlooked something.

Thanks!

Elysia · 07-12-2009, 02:15 PM

I see no problem with this except for a few minor things:
- It's clearer if you add virtual to every function that is virtual. Every function that overrides a virtual function is a base class is considered a virtual function, but it isn't obvious right away unless you can see the base class.
- Don't use printf, use cout.

Aside from that, could you provide the smallest simplest code that demonstrates the problem?

**Fordy** · 07-12-2009, 02:19 PM

They are all private, how are they getting called?

>>Don't use print, use cout.

printf is still valid. Please dont bother giving a lecture about type safety.

tabstop · 07-12-2009, 02:20 PM

According to my compiler, neither MyFunc nor MyFunc2 are overloaded; code such as

Code:

    FooBar object;
    Foo* ptr = &object;
    object.myFunc2();
    ptr->myFunc2();
    object.myFunc();
    ptr->myFunc();

does not of course work, since all the functions are private, but the first ones both resolve to FooBar::myFunc[2] and the second ones both resolve to Foo::myFunc[2], not FooBar. (Edit: by "first ones" I mean object., and by "second ones" I mean ptr->.)

leeor_net · 07-12-2009, 02:36 PM

Quote:

Don't use print, use cout.

Yes, I know that. This was a quick and dirty example to illustrate a point.

The code that I'm having trouble with is big and complex. But I'll strip out the stuff that isn't necessary. Here goes:

Code:

class Control
{
public:
	Control();
	void update();

	addControl(Control *);
	update();
private:
	virtual void draw();
	virtual void initialize();
};


class Window : public Control
{
public:
	Window();
	~Window();

private:
	void draw();
	void initialize();

	std::string	mTitlebar;
	int		mTitlebarHeight;
};


class DebugWindow : public Window
{
public:
    DebugWindow();
    ~DebugWindow();

private:
	void initialize();
};

In particular the trouble shows up when I run the addControl() function:

Code:

void Control::addControl(Control *control)
{
	mChildControls.push_back(control);
	mChildControls.back()->initialize();
}

mChildControls.back()->initialize() only ever gets called once and only runs the initialize() function from the class Control, not Window or DebugWindow.

The update function looks like this:

Code:

void Control::update()
{
	draw();

	for(unsigned int i = 0; i < mChildControls.size(); i++)
		mChildControls[i]->update();
}

This function tells the Control to draw itself and then to iterate through all of its children and tells them to update themselves which in turn calls their update and so on... Now, this works. The correct draw() function is called.

I may have just answered my own question as to why so now the question has turned into how. But just in case, I'm still asking...

mChildControls is defined as a vector<Control*>. There are probably a few things here and there that don't appear to be defined but just note that I've stripped out all of the irrelevant functions/variables/etc.

Elysia · 07-12-2009, 02:41 PM

Quote:

Originally Posted by Fordy

>>Don't use print, use cout.
printf is still valid. Please dont bother giving a lecture about type safety.

But should be avoided in C++, unless you have a very good reason.
Sure, go ahead and use printf and other C-family functions, but chances are it will become "C+" code instead of C++ code then.

Oh and consider that private functions aren't inherited, so it may just be that the functions are hidden instead of overloaded. It didn't occur to me before. Perhaps you want to use protected?
I am also going to have to rant a little about not removing parameter names.

leeor_net · 07-12-2009, 02:45 PM

I;m not one to complain but can we keep the irrelevant chatter out of this topic? This isn't a debate about coding style, appropriateness of function calls, etc.

I tried moving around the functions to protected and public but no game. Clearly I've misunderstood something.

Thank you.

tabstop · 07-12-2009, 02:55 PM

I'm not sure I've got the hang of what your problem is. I've modified your code so that it makes sense and whatever:

Code:

#include <cstdio>
#include <string>
#include <vector>

class Control
{
public:
	void update();

	void addControl(Control *);
private:
    std::vector<Control *> mChildControls;
	virtual void draw();
	virtual void initialize();
};

void Control::draw() {
    printf("Control::draw\n");
}

void Control::initialize() {
    printf("Control::initialize\n");
}

class Window : public Control
{
public:

private:
	void draw();
	void initialize();

	std::string	mTitlebar;
	int		mTitlebarHeight;
};

void Window::draw() {
    printf("Window::draw\n");
}

void Window::initialize() {
    printf("Window::initialize\n");
}


class DebugWindow : public Window
{
public:

private:
	void initialize();
};

void DebugWindow::initialize() {
    printf("DebugWindow::initialize\n");
}

void Control::addControl(Control *control)
{
	mChildControls.push_back(control);
	mChildControls.back()->initialize();
}

void Control::update()
{
	draw();

	for(unsigned int i = 0; i < mChildControls.size(); i++)
		mChildControls[i]->update();
}

int main() {
    Window foo;
    DebugWindow bar;
    Control bob;
    bob.addControl(&foo);
    bob.addControl(&bar);
    return 0;
}

This prints

Code:

Window::initialize
DebugWindow::initialize

as you probably want. Can you modify this to show what's going on?

**Fordy** · 07-12-2009, 02:56 PM

Quote:

Originally Posted by leeor_net

I;m not one to complain but can we keep the irrelevant chatter out of this topic? This isn't a debate about coding style, appropriateness of function calls, etc.

I know mate. Some people think that by pointing out pointless things like that they are helping.

Not wanting to ask an obvious question, but when you pass a control to addControl, you are passing a Window or DebugWindow via a Control pointer and not just a plain old Control object are you?

leeor_net · 07-12-2009, 03:13 PM

Nope, was definitely passing in a DebugWindow object.

Now here's the funny thing. I figured out my problem and I really could slap myself.

The way I have everything set up, I have what I call a Top Level or Root control. When I created DebugWindow, I... forgot to call addControl() to the root... <sigh>. It never occured to me that that could be the problem.

I hope I haven't wasted anybody's time. I do appreciate the comments and the suggestions...

Thanks again!

cpjust · 07-12-2009, 08:16 PM

You aren't "overloading" any functions, you're "overriding" them. Overloading has nothing to do with inheritance.

leeor_net · 07-13-2009, 06:37 AM

Yes, I realize that. Thank you for the vocabulary lesson...

07-12-2009, 02:09 PM	#1
leeor_net Professional Chef Join Date: Apr 2004 Location: Charles Town, WV Posts: 144	[solved] Base Class Overloaded Virtual Function Hello again. I've been working with a somewhat complex class heirarchy and have run into a problem. More than likely it stems from not fully understanding how C++ derived classes inherit from base classes. To keep things simple, I have the following three classes defined as such: Code: class Foo { private: virtual void myFunc() { printf("Hello!"); } virtual void myFunc2() { printf("'ello!"); } }; class Bar : public Foo { private: void myFunc() { printf("Hello 2!"); } void myFunc2() { printf("'ello 2!"); } }; class FooBar : public Bar { private: void myFunc() { printf("Hello 3!"); } void myFunc2() { printf("'ello 3!"); } }; The weird thing that's happening is that when I call the overloaded function from FooBar myFunc() from a list of Foo's (e.g., std::vector<Foo>), the correct function is called. But myFunc2() calls the function from Foo, not from Bar or FooBar. I'm not sure this is clear enough but I'm hoping I provided enough information and that my problem is something really simple or that I just overlooked something. Thanks! __________________ - Leeor Last edited by leeor_net; 07-12-2009 at 03:38 PM. Reason: Problem solved.*
leeor_net is offline

07-12-2009, 02:19 PM	#3
Fordy &TH of undefined behavior Join Date: Aug 2001 Posts: 5,261	They are all private, how are they getting called? >>Don't use print, use cout. printf is still valid. Please dont bother giving a lecture about type safety. __________________ "If A is success in life, then A equals x plus y plus z. Work is x; y is play; and z is keeping your mouth shut." Albert Einstein (1879 - 1955) Board Rules
Fordy is offline

07-12-2009, 02:20 PM	#4
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 10,163	According to my compiler, neither MyFunc nor MyFunc2 are overloaded; code such as Code: FooBar object; Foo* ptr = &object; object.myFunc2(); ptr->myFunc2(); object.myFunc(); ptr->myFunc(); does not of course work, since all the functions are private, but the first ones both resolve to FooBar::myFunc[2] and the second ones both resolve to Foo::myFunc[2], not FooBar. (Edit: by "first ones" I mean object., and by "second ones" I mean ptr->.)
tabstop is offline

07-12-2009, 02:45 PM	#7
leeor_net Professional Chef Join Date: Apr 2004 Location: Charles Town, WV Posts: 144	I;m not one to complain but can we keep the irrelevant chatter out of this topic? This isn't a debate about coding style, appropriateness of function calls, etc. I tried moving around the functions to protected and public but no game. Clearly I've misunderstood something. Thank you. __________________ - Leeor Last edited by leeor_net; 07-12-2009 at 02:50 PM.
leeor_net is offline

07-12-2009, 03:13 PM	#10
leeor_net Professional Chef Join Date: Apr 2004 Location: Charles Town, WV Posts: 144	Nope, was definitely passing in a DebugWindow object. Now here's the funny thing. I figured out my problem and I really could slap myself. The way I have everything set up, I have what I call a Top Level or Root control. When I created DebugWindow, I... forgot to call addControl() to the root... <sigh>. It never occured to me that that could be the problem. I hope I haven't wasted anybody's time. I do appreciate the comments and the suggestions... Thanks again! __________________ - Leeor
leeor_net is offline

07-12-2009, 08:16 PM	#11
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,425	You aren't "overloading" any functions, you're "overriding" them. Overloading has nothing to do with inheritance. __________________ "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008 "the internet is a scary place to be thats why i dont use it much." - billet, 03/17/2010
cpjust is offline

07-13-2009, 06:37 AM	#12
leeor_net Professional Chef Join Date: Apr 2004 Location: Charles Town, WV Posts: 144	Yes, I realize that. Thank you for the vocabulary lesson... __________________ - Leeor Last edited by leeor_net; 07-13-2009 at 07:19 AM.
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