Member functions as callbacks

[Leroy1981]

When I try to use a typical function as a callback everything works fine but if i try it with a member function of a class that has the same return type and parameter list i get a cannot cast from (classname: (function parameters) to (function parameters).

the borland error code is e2031 if that helps.

 

[Cagliostro]

make it static.

Ion Filipski

 

[bNasty]

Problem is that one more parameter is passed to every member-function. It's a this-pointer which is hidden from you, but will spoil argument types and order, so that's why you CAN NOT use member-functions as callbacks.
Use static or global fn. instead.

 

[chipperMDW]

You can most certainly use (non-static) member functions as callbacks. You have to pass the object you're calling it on, of course.

The STL has some stuff in the <functional> header for doing just that, check out something called mem_fun (I believe) in some STL reference libraries.

Also, Boost (

http://www.boost.org)

has a callback function library that's kind of in flux right now, but will become part of the next C++ Standard (in some form or another). It has member function callback capability.

You could, of course, just use a function object containing a reference to the object whose member function you want to call.

 

[Cagliostro]

by the way, If you want to make a member WndProc, you can attach a pointer by doing:

class xxx
{
static LRESULT CALLBACK WndProc(HWND, int, WPARAM, LPARAM);
LRESULT WndProc(int msg, WPARAM wParam, LPARAM lParam)
{
handle messages there
....
}
};
xxx
{
static LRESULT CALLBACK WndProc(HWND, int, WPARAM, LPARAM);
};
LRESULT CALLBACK XXX::WndProc(HWND hWnd, int msg, WPARAM wParam, LPARAM lParam)
{
xxx* mObj = (xxx*)GetWindowLong(hWnd, GWL_USERDATA);
if(mObj)return mObj->WndProc(msg, wParam, lParam);
return DefWindowProc(hWnd, msg, wParam, lParam)

}

......
create window there

xxx myXxx;
hWnd = .......
SetWindowLong(hWnd, GWL_USERDATA, (LONG)&myXxx);



Ion Filipski