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Wrapper C++ classes for API's with callbacks.
Posted on 2000-03-09
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.
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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7 Comments
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.
example follows.
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;
}
};
{
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;
}
};
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.
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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