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ATL/COM function pointer problem

Started by timmiepoo Jan 6, 2003 at 12:40 PM 0 replies 800+ views

Original Post

Jan 6, 2003 at 12:40 PM

hello all. allow me to preface this post by saying that i am by no means an ATL/COM expert -- not even close. i''m in a situation where i have to figure this out as i go. i have some good reference material, but i''m stuck. hopefully one of you gurus can help me out. here goes... i''m in the process of wrapping an existing C API in COM server that will be called from ASP pages. there is one interface, ITask, exposing an Execute() method, which is the primary workhorse. from inside this Execute() function, i make calls to the afore-mentioned API. one of these API functions expects a function pointer as a parameter, like so:

hr = p4SetOutputFunction (sessionHandle, //long
						processReportData) //function pointer

the p4SetOutputFunction() uses the function pointer param to set up a callback to handle each line of report output. for instance, if the API were used in a Windows console app, the processReportData() function would print each line of report output to stdout. now here is my problem. p4SetOutputFunction() and processReportData() are now contained in my COM Interface. a function pointer to processReportData() in C++ now also passes a hidden pointer to this (the CTask object -- my implementation of the ITask interface). but i can''t modify the arg list for p4SetOutputFunction(), because it''s part of an existing API. so i''m stuck trying to figure out how to pass the function pointer. sorry for the long-winded explanation. if it''s still clear as mud, please let me know and i''ll try to explain a little better. i''m doing the best i can at explaining with my limited knowledge. i''ll be grateful for any suggestions. thanks!

timmiepoo

Author

timmiepoo

Jan 8, 2003 at 9:24 AM

posting some source code to hopefully clear up my terrible explanation...

My COM object has one Interface, ITask, implemented by the CTask class. It currently exposes one method, Execute():

  
STDMETHODIMP CTask::Execute(BSTR bstrTaskName, BSTR bstrMachine, BSTR bstrCoDiv, BSTR bstrOperId, 
							BSTR bstrOperPassword, BSTR bstrSystemUserId, BSTR bstrSystemPassword, 
							BSTR bstrEnvironment, int iPort, BSTR bstrParams, BSTR *pbstrOutput)
{
}

Inside this method, i make a call to an API function. Not the Win32 API. Another API that our primary business software uses. It''s a DLL written by another company. The call looks like this:

  
p4SetOutputFunction (sessionHandle, lpFunc);

The second parameter is the problem. The API expects a function pointer to use as a callback for writing report data line-by-line. But the function I need to use is a member of CTask. So, the API is expecting this:

(*)(long,int,int,int,int,const char *)

but i''m passing it this:

(CTask::*)(long,int,int,int,int,const char *)

i''ve tried various ways of casting the pointer, including the following:

  
// global function pointer declaration, not a member of CTask

int (*lpFunc)(long,int,int,int,int,const char *);

STDMETHODIMP CTask::Execute(BSTR bstrTaskName, BSTR bstrMachine, BSTR bstrCoDiv, BSTR bstrOperId, 
							BSTR bstrOperPassword, BSTR bstrSystemUserId, BSTR bstrSystemPassword, 
							BSTR bstrEnvironment, int iPort, BSTR bstrParams, BSTR *pbstrOutput)
{
	// point global function pointer to my CTask function

	lpFunc = CTask::processReportOutputData;
}

i''m still getting this error:

error C2440: ''='' : cannot convert from ''int (__thiscall CTask::*)(long,int,int,int,int,const char *)'' to ''int (__cdecl *)(long,int,int,int,int,const char *)''
There is no context in which this conversion is possible

anyone know how i can make this work, without rewriting the existing API to accept a class function pointer? i''m trying to figure out (1) if it''s possible, and (2) how to do it. if it''s not possible, i can find out if i can modify and recompile the DLL. right now i''m just trying to figure out if there is an alternative, since i can''t seem to figure it out. thanks!

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