Programming Forums > Application Development > C++

Lakrids · Feb 9th, 2008, 12:11 PM

Hello,

There is a quite problematic issue I would like to have solved.

Consider an event handling function for a button. This function takes, among it's arguments, a pointer to a function, which it will execute if the button is pressed. Let's say I have a number of objects of class C, and I would like to do an operation on one of them if the button is pressed. I cannot pass "&object.operation" as a parameter to the button handling function, because the c++ standard doesn't allow that. Instead, I have to pass "&C::action". But passing this will not modify the one object I want to have modified when the user clicks on the button.

Is there a way to pass a function pointer towards the method of THE object I want to modify?

Thank you

Klarre · Feb 9th, 2008, 1:04 PM

Unless the function you will call isn't static you have to pass a pointer to the object too. Something like this maybe. This code may not compile but you probably get the idea. Not sure about the member function syntax.

(Toggle Plain Text)

class ButtonCallbackListener
{
public:
};

class Button
{
public:
	Button(ButtonCallbackListener* object, void (ButtonCallbackListener::*functionPtr)());

	void onPressed() { mObject->(*mFunctionPtr)(); }

private:
	ButtonCallbackListener* mObject;
	void (ButtonCallbackListener::*mFunctionPtr)();
};

class MyClass : public ButtonCallbackListener
{
public:
	MyClass() { mButton = new Button(this, &MyClass::callback); }

	void callback() {}

private:
	Button* mButton;
};

Lakrids · Feb 9th, 2008, 4:51 PM

Thank you for your reply. However, it's not exactly what I was looking for. I don't think I was clear enough in my question, so here's a simple example :

 cplusplus Syntax (Toggle Plain Text) 
class Button {
    public :
        Button(etc, etc, etc, void (*action)());
        void onPressed() { *action(); }
};
 
void randomAction() {
    // Some code
}
 
int main() {
 
    ObjectA A;
    ObjectA AA;
    ObjectB B;
    ObjectC C;
 
    Button b(etc, etc, etc, &A.action); // isn't allowed by C++ ISO
    Button b(etc, etc, etc, &B.action); // isn't allowed by C++ ISO
    Button b(etc, etc, etc, &randomAction); // is allowed by C++ ISO
 
    // Other code
 
    return 0;
}
class Button {
    public :
        Button(etc, etc, etc, void (*action)());
        void onPressed() { *action(); }
};

void randomAction() {
    // Some code
}

int main() {

    ObjectA A;
    ObjectA AA;
    ObjectB B;
    ObjectC C;

    Button b(etc, etc, etc, &A.action); // isn't allowed by C++ ISO
    Button b(etc, etc, etc, &B.action); // isn't allowed by C++ ISO
    Button b(etc, etc, etc, &randomAction); // is allowed by C++ ISO

    // Other code

    return 0;
}

So what I want to do, is execute an action ON ANY KIND OF SPECIFIC OBJECT i want, when the button is pressed. Which is not allowed by the C++ ISO, which says :

Quote:

error : ISO C++ forbids taking the address of a bound member function to form a pointer to member function. Say ‘&ObjectA::action’

But saying "&ObjectA::action" WILL NOT alter the specific object ("A" in this case), it will only call ObjectA::action(), not A.action().

Hope you guys understand better now

Any suggestions?

grumpy · Feb 9th, 2008, 7:41 PM

Klarre understood you correctly; you're trying to do something invalid. A pointer to member function does not carry any information about what instance of the class it will act on (i.e. the object), so you need to pass that information separately.

Lakrids · Feb 10th, 2008, 5:14 AM

Yes, but the thing is, I don't want to have an ObjectA or an ObjectB as private members of the Button class. The code must work for any kind of object there is. In other terms, the button handling code must not know which object it will be performing an operation on. It could be an int, a Dog, a Cat, or another Button.

grumpy · Feb 10th, 2008, 5:43 AM

Who says you have to? Either pass two arguments, or pass a structure that contains both pieces of information (the address of whatever you need to act on, and the function that acts on it). You're already passing one of those bits of information to your Button class (and presumably storing it somewhere, such as with a member of the Button) so why not pass the complete set of information needed?

If I asked you to put fruit into a jar, you'd expect me to tell you where the fruit is and where the jar is. If I only tell you where the fruit is, you probably won't succeed in putting it into the jar. You're trying to give your software (your Button class) the fruit, while throwing the jar away.

Lakrids · Feb 10th, 2008, 6:03 AM

You're right! Thanks! So what you suggest, is to have some kind of CallbackListener class from which the objects I want to manipulate inside the button handling code inherit? That would solve my problem. But, in a more general way, how would you do it? I mean to pass the address of an int for example, which is does not inherit from a CallbackListener class...the Button class does not know what object it will receive as an argument...unless the Butoon class is a template class. But let's not go that far

EDIT : removed unnecesary content.

Lakrids · Feb 10th, 2008, 11:16 AM

OK, problem solved with static member functions. But problem persists with non-static ones

Any ideas?

Klarre · Feb 10th, 2008, 12:32 PM

You have already gotten the answer. It is impossible to do what you want to do by using C++.

Lakrids · Feb 10th, 2008, 4:55 PM

Quote:

It is impossible to do what you want to do by using C++.

Lol thanks!

But a fellow programmer just pointed me out that in fact, it is possible. And he wrote me code which works. The idea is to make a static wrapper member function which will call the non-static one. It also involves passing a pointer to the object, which is then converted into a void*, and in the member function itself, reconverted into the class's type, which will then be passed to the function as the first argument, the implicit "this". This way the wanted action is performed on the object in question, and always gives accurate results.

Remember, this is C++. Nothing is impossible.

Feb 9th, 2008, 12:11 PM	#1
Lakrids Newbie Join Date: Dec 2007 Posts: 28 Rep Power: 0	Pointer to member function Hello, There is a quite problematic issue I would like to have solved. Consider an event handling function for a button. This function takes, among it's arguments, a pointer to a function, which it will execute if the button is pressed. Let's say I have a number of objects of class C, and I would like to do an operation on one of them if the button is pressed. I cannot pass "&object.operation" as a parameter to the button handling function, because the c++ standard doesn't allow that. Instead, I have to pass "&C::action". But passing this will not modify the one object I want to have modified when the user clicks on the button. Is there a way to pass a function pointer towards the method of THE object I want to modify? Thank you

Feb 9th, 2008, 1:04 PM	#2
Klarre Game engine designer Join Date: May 2005 Location: Sweden Posts: 301 Rep Power: 4	Re: Pointer to member function Unless the function you will call isn't static you have to pass a pointer to the object too. Something like this maybe. This code may not compile but you probably get the idea. Not sure about the member function syntax. (Toggle Plain Text) class ButtonCallbackListener { public: }; class Button { public: Button(ButtonCallbackListener* object, void (ButtonCallbackListener::functionPtr)()); void onPressed() { mObject->(mFunctionPtr)(); } private: ButtonCallbackListener* mObject; void (ButtonCallbackListener::mFunctionPtr)(); }; class MyClass : public ButtonCallbackListener { public: MyClass() { mButton = new Button(this, &MyClass::callback); } void callback() {} private: Button mButton; }; class ButtonCallbackListener { public: }; class Button { public: Button(ButtonCallbackListener* object, void (ButtonCallbackListener::functionPtr)()); void onPressed() { mObject->(mFunctionPtr)(); } private: ButtonCallbackListener* mObject; void (ButtonCallbackListener::mFunctionPtr)(); }; class MyClass : public ButtonCallbackListener { public: MyClass() { mButton = new Button(this, &MyClass::callback); } void callback() {} private: Button mButton; }; __________________ http://www.klarre.se

Feb 9th, 2008, 7:41 PM	#4
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,223 Rep Power: 5	Re: Pointer to member function Klarre understood you correctly; you're trying to do something invalid. A pointer to member function does not carry any information about what instance of the class it will act on (i.e. the object), so you need to pass that information separately.

Feb 10th, 2008, 5:14 AM	#5
Lakrids Newbie Join Date: Dec 2007 Posts: 28 Rep Power: 0	Re: Pointer to member function Yes, but the thing is, I don't want to have an ObjectA or an ObjectB as private members of the Button class. The code must work for any kind of object there is. In other terms, the button handling code must not know which object it will be performing an operation on. It could be an int, a Dog, a Cat, or another Button.

Feb 10th, 2008, 5:43 AM	#6
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,223 Rep Power: 5	Re: Pointer to member function Who says you have to? Either pass two arguments, or pass a structure that contains both pieces of information (the address of whatever you need to act on, and the function that acts on it). You're already passing one of those bits of information to your Button class (and presumably storing it somewhere, such as with a member of the Button) so why not pass the complete set of information needed? If I asked you to put fruit into a jar, you'd expect me to tell you where the fruit is and where the jar is. If I only tell you where the fruit is, you probably won't succeed in putting it into the jar. You're trying to give your software (your Button class) the fruit, while throwing the jar away.

Feb 10th, 2008, 6:03 AM	#7
Lakrids Newbie Join Date: Dec 2007 Posts: 28 Rep Power: 0	Re: Pointer to member function You're right! Thanks! So what you suggest, is to have some kind of CallbackListener class from which the objects I want to manipulate inside the button handling code inherit? That would solve my problem. But, in a more general way, how would you do it? I mean to pass the address of an int for example, which is does not inherit from a CallbackListener class...the Button class does not know what object it will receive as an argument...unless the Butoon class is a template class. But let's not go that far EDIT : removed unnecesary content. Last edited by Lakrids; Feb 10th, 2008 at 6:15 AM.

Feb 10th, 2008, 11:16 AM	#8
Lakrids Newbie Join Date: Dec 2007 Posts: 28 Rep Power: 0	Re: Pointer to member function OK, problem solved with static member functions. But problem persists with non-static ones Any ideas?

Feb 10th, 2008, 12:32 PM	#9
Klarre Game engine designer Join Date: May 2005 Location: Sweden Posts: 301 Rep Power: 4	Re: Pointer to member function You have already gotten the answer. It is impossible to do what you want to do by using C++. __________________ http://www.klarre.se

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Programming Forums > Application Development > C++

Pointer to member function