quick help with non static callback

hi im using the following code to demo a callback:
how can i change the function _renderCallback to become non static?

thanks.

Code:

---

#include <iostream>

using namespace std;

class MyClass
{
public:

    MyClass()

    {}

    ~MyClass() {}

    static void _renderCallback(void)

    {
        cout << "You clicked." << std::endl;
    }
};

//------------------------------------------------


typedef void(*t_CallBackFunction)() ;

t_CallBackFunction theCallbackFunc ;


void callback_registration_function( t_CallBackFunction func )

{
    theCallbackFunc = func ;
}

void trigger_callback()

{
    theCallbackFunc();
}

//------------------------------------------------

int main ()
{
    MyClass obj ;

    callback_registration_function( MyClass::_renderCallback ) ;

    trigger_callback() ;

    return 0;
}

---

You don't. (Non-static member functions require an object to call them with, which your trigger is not going to have available.)

A member function is not an ordinary function, by any means. First of all, it requires an *implicit* pointer to 'this'. Second, the function may well be virtual, in which case the compiler can't simply map it to a physical address at compile time.

To get that kind of functionality, you're going to need to use a functor (it's a fairly involved idiom, which is why I posted a link instead of an example).

What you can do is pass a class pointer instead of a function pointer, then just call a member function of that class. That is the C++ way after all.

thanks all. im building a new gui and need to know the best way to create a callback to a member function. any ideas other than a functors, it needs to be short and clean.

Quote:

---

Originally Posted by bithub

What you can do is pass a class pointer instead of a function pointer, then just call a member function of that class. That is the C++ way after all.

---

how do you pass a class pointer?

i think ill create a bass class for all my gui and use a class pointer. thanks all.

i have a problem. i cant use a class pointer as i need a function pointer.

my classes need functions like playButtonPressed and volumeControlChanged etc.

any ideas?

Can the callback store user defined data? What framework are you using?

Quote:

---

Originally Posted by bithub

Can the callback store user defined data? What framework are you using?

---

im using pure c++, no framework to create my gui, im using opengl

I need a simple way to pass in a pointer to a non static member function.

If you aren't using an existing framework, why can't you use a class pointer instead of a function pointer?

Quote:

---

Originally Posted by bithub

If you aren't using an existing framework, why can't you use a class pointer instead of a function pointer?

---

im a little confused by using a class pointer?

my gui needs functions like playButtonClicked(bool clicked) and masterVolumeChanged(float level)

if i use a class pointer? how can i use different functions within my gui

So have a linked list of pointers to objects that share a common virtual base class. It's not as clean as the functor approach, but perhaps less complicated. If not that, you could always do things "the C way", eg: static global variables, but then that's going to be even less flexible.

Whenever you have old C-style callbacks (which are standalone/static, don't have class objects) the equivalent C++-style would be std::function/std::bind (or boost::function/boost::bind). Simply replace these two lines:

Code:

---

typedef void(*t_CallBackFunction)() ;

---

Code:

---

callback_registration_function( MyClass::_renderCallback ) ;

---

with these:

Code:

---

typedef boost::function<void()> t_CallBackFunction ;

---

Code:

---

callback_registration_function(boost::bind(&MyClass::_renderCallback, obj) ;

---

Make _renderCallback non-static, and include <boost/function.hpp> and <boost/bind.hpp> (have to download from boost.org). If you go with this, you should read the documentation.