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Understanding Function Pointers and other such things (C++)


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#1 Plethora   Members   -  Reputation: 679

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Posted 03 April 2013 - 01:35 PM

I recently started messing around with the SFGUI library, its basically a gui library based in SFML.  On its own, I was able to follow the tutorial here:  http://sfgui.sfml-dev.de/p/docs/guide/hello_world Without a problem.  When I went about adding it to my own program, though, I ran into an issue with this line:

 

button->GetSignal( sfg::Button::OnLeftClick ).Connect( &OnButtonClick );
 

Now, in my program I'm attempting to put all of this within a class called Gui_Manager.  I have a class method called OnButtonClick(), however as I've learned I cannot use a function pointer like the one in the tutorial code with a bound member function.  So I consulted the documentation and found that Connect has two declarations as follows:

 

unsigned int 	Connect (Delegate delegate)
unsigned int 	Connect (void(Class::*function)(), Class *object)

 

The tutorial uses the first constructor with the Delegate being a function pointer (&OnButtonClick).  I'm guessing as I want to connect with a class method, I now want to use the second declaration, however, I'm not even sure how I would call that (I've randomly tried some things with no results).  So I have two questions to anyone who would be so kind...

 

1)  An example of how I would call Connect using the second definition... I feel like just having some code to point me in the right direction would be lovely...

 

2)  I could fairly easily create the required function callbacks outside of my class, but doing so goes against most of the Object Oriented conditioning I've been through in my life.  Would that be an acceptable solution?  Or are there notably drawbacks to doing things that way when every other spec of code in my program is contained in a class?

 

Thanks for any help you can give!  :)


I'm working on a game!  It's called "Spellbook Tactics".  I'd love it if you checked it out, offered some feedback, etc.  I am very excited about my progress thus far and confident about future progress as well!

 

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#2 Juliean   GDNet+   -  Reputation: 2720

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Posted 03 April 2013 - 01:41 PM

Did you try

button->GetSignal( sfg::Button::OnLeftClick ).Connect( &OnButtonClick, this );

? Obviously you need to provide a function pointer as first and the actual object as the second paramter, since the function pointer does not store an actual object to call the function on.


Edited by The King2, 03 April 2013 - 01:42 PM.


#3 Plethora   Members   -  Reputation: 679

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Posted 03 April 2013 - 01:45 PM

Well... I did try your suggestion, and it didn't work, however this:

 

 

button->GetSignal( sfg::Button::OnLeftClick ).Connect( &Gui_Manager::OnButtonClick, this );
 

Did work.  

 

I think I was definitely making this out to be more complicated than it needed to be, thanks for the inspiration!

 

:)


I'm working on a game!  It's called "Spellbook Tactics".  I'd love it if you checked it out, offered some feedback, etc.  I am very excited about my progress thus far and confident about future progress as well!

 

http://infinityelephant.wordpress.com


#4 GeneralQuery   Crossbones+   -  Reputation: 1263

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Posted 03 April 2013 - 01:55 PM

I know nothing of SFML so all I can offer is some background info for you. The signature looks like the common solution to having class methods as callbacks, i.e. you declare a static method inside said class (which does not take the implicit this pointer as it's first argument) which takes a pointer to an object of said class as an argument. The static method then calls the appropriate method of the object in question.

Your static method would look something like this:

class MyClass
{
public:

static void OnClickCallback(MyClass* obj) {obj->OnClick();}

// and so on

};

You would register the callback like this: Connect(&MyClass::OnClickCallback, this) from some (non-static) method in your class (or elswhere in your code, just replace the this pointer with a pointer to the object you wish to connect).


Edit: whoops, looks like a flurry of posts since I started my reply and you have the problem solved. My bad!
Edit2: and I messed up the code tags, fixed!

Edited by GeneralQuery, 03 April 2013 - 01:58 PM.


#5 Paradigm Shifter   Crossbones+   -  Reputation: 5433

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Posted 03 April 2013 - 02:00 PM

Nope, it's a pointer to member function, not static, because of this syntax

 

void(Class::*function)()

 

which needs an object to be called, like so

 

obj->*function();

 

EDIT: Or obj.*function(); if called through a non-pointer

 

That's why the second method takes the this pointer (to know which object to call it with).

 

If it was a normal C compatible function pointer it would be declared like this

 

void (*function)();

 

and then it would require a static member function (but then you'd want to pass it a pointer to the object, so it would have a pointer as first argument most likely).


Edited by Paradigm Shifter, 03 April 2013 - 02:01 PM.

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#6 Juliean   GDNet+   -  Reputation: 2720

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Posted 03 April 2013 - 02:31 PM

Well... I did try your suggestion, and it didn't work, however this:

 

Damn, missed the "&" before the function, this happens every now and then :/ You might probably leave out the Gui_Manager:: before the function name, could save you a little typing, as long as you "connect" from inside another function of the same class.


Edited by The King2, 03 April 2013 - 02:32 PM.





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