Re: a callback function across the dll boundary
On Fri, 23 Jun 2006 19:16:11 -0700, JD <jdt_young@yahoo.com> wrote:
Hi,
I have two classes CMain and CSupport that are located in two different
dlls, main.dll and support.dll, respectively. main.dll depends on
support.dll. I want that CMain gets notified during CSupport's
destruction (it's CSupport rather than CMain that knows when CSupport
needs to be destructed). We can use MFC's message notification
mechanism. But would it be possible to simply pass a function in CMain
to CSupport as a callback routine? Can callback functions go across the
dll boundary?
Yes. Examples include window procedures, callbacks for EnumWindows, and on
and on. It's extremely common. By and large, the significant DLL boundary
concerns data, not code. Data becomes a problem when two DLLs have their
own private copies of a given piece of data, when they expect to be sharing
the same copy; examples include the heap when linking to different CRTs and
template static data when a template specialization that contains static
data is instantiated in two different DLLs. Code becomes a problem when a
DLL is unloaded and someone tries to call the unloaded code.
The following code illustrates what I want, but it must
have syntax errors. Can someone correct the syntax? Any help is much
appreciated.
// ******************
In main.dll:
class CMain
{
..
void CallBackFunc(int nRet);
static void CallBackFunc(int nRet);
(That this needs to be static follows from what you go on to try.)
};
CMain m;
..
CSupport *m_pSupport = new CSupport(m.CallBackFunc());
CSupport *m_pSupport = new CSupport(&CMain::CallBackFunc);
// ******************
In support.dll
#ifdef SUPPORTDLL
#define CLASS_DECL _declspec( dllexport )
#else
#define CLASS_DECL _declspec( dllimport )
#endif
typedef void(*funcptr)(int);
class CLASS_DECL CSupport
{
CSupport(funcptr *p)
{
m_p = p;
}
~CSupport()
{
if (m_p)
m_p(3); // return code is 3
}
funcptr m_p;
};
Just be sure the DLL containing CMain::CallbackFunc outlives the modules
containing the CSupport instances; you really, really don't want to call
into unloaded DLLs.
--
Doug Harrison
Visual C++ MVP