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Wrapper C++ classes for API's with callbacks.

Posted on 2000-03-09
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Last Modified: 2010-04-02
Hi.
Question: What's the best way to wrap an API call that receives a callback function pointer in a C++ class.

I'm asking because:
1. Callback functions have to be static.
2. They need to access the members of the class in which they reside. In order for them to do that, these have to be static as well...

I tried taking the callbacks outside the class definition, but that's plain ugly. It worked though - But I had to pass them a pointer to my class, which meant they could only access the public members...

Is there a smarter way of solving this?
Thanks,
Assaf.
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Question by:AssafLavie
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7 Comments
 
LVL 5

Expert Comment

by:Wyn
ID: 2602096
set it as friend function
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Accepted Solution

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nietod earned 200 total points
ID: 2602098
Most windows callback function are passed a VOID * pointer that is specified when the callback is "registered".  You can use this VOID * to specify a pointer to an object whose member function is called.  The static callback function can convert this VOID * pointer back to a  pointer to an object and then call the member function.

example follows.
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LVL 22

Expert Comment

by:nietod
ID: 2602119
class EnmWnd
{
   static BOOL CALLBACK EnmPrc(HWND WndHnd,LPARAM LngPrm)
   {
      EnmWnd *ObjPtr = (EnmWnd *)LngPrm; // Convert back to a object pointer.
      return ObjPtr->WndFnd(WndHnd); // Call member procedure.
   }
public:
   void Enumerate()
   {
      EnumWindows((WNDENUMPROC) EnmPrc,(LPARAM) this);
   }
   BOOL WndFnd(HWND WndHnd)
   {
//        Do something with the window handle and this object.
       return TRUE;
   }
};
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LVL 4

Author Comment

by:AssafLavie
ID: 2602367
Thanks.
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LVL 4

Author Comment

by:AssafLavie
ID: 2602373
But what should I do when I cannot pass an application defined value to the callback?
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LVL 22

Expert Comment

by:nietod
ID: 2602971
Redesign.

You might be able to use a global variable to store a pointer to the object.  (Assuiming you don't have to worry about multi-threadding.)

But honestly, I dont't think that will ever be a problem.  I can't think of any that doesn't take a parameter like this.
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LVL 4

Author Comment

by:AssafLavie
ID: 2603937
Thanks again.
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