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data conversion error when assigning function to function pointer

Posted on 2007-03-21
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Last Modified: 2013-12-08
Hi Experts,
    I am trying to assign a function to a function pointer member in CFactoryTemplate, which is a class from directshow SDK

this is the class that i wrote:
class CMYFilter: public CTransInPlaceFilter
{
    public:
        CUnknown * WINAPI CreateInstance(LPUNKNOWN pUnk, HRESULT *pHr);
};

What i am trying to do is assign CreateInstance to a member of type
LPFNNewCOMObject
which has its original declaration as
typedef CUnknown *(CALLBACK *LPFNNewCOMObject)(LPUNKNOWN pUnkOuter, HRESULT *phr);

the assignment is shown here:
CFactoryTemplate g_Templates[1] =
{
  {
    g_wszName,
    &CLSID_CMYFilter,
    CMYFilter::CreateInstance,  // line with error
    NULL,
    NULL
  }
};

I kept receiving this error message at the line CMYFilter::CreateInstance.

error C2440: 'initializing' : cannot convert from 'CUnknown *(__cdecl
CMYFilter::* )(LPUNKNOWN,HRESULT *)' to 'LPFNNewCOMObject'
1>        None of the functions with this name in scope match the target type

This is very strange since all the sample codes, including those came with
SDK, were written like this.

Is there any reason why this is happening?
thanks!
0
Comment
Question by:fungi8210
  • 5
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7 Comments
 
LVL 86

Accepted Solution

by:
jkr earned 500 total points
Comment Utility
You need to make that function a 'static' member of your class - non static ones are passed an invisible 'this' pointer which makes the signature different, thus the error.

class CMYFilter: public CTransInPlaceFilter
{
    public:
        static CUnknown * WINAPI CreateInstance(LPUNKNOWN pUnk, HRESULT *pHr);
};

will get that to work.
0
 
LVL 86

Expert Comment

by:jkr
Comment Utility
BTW, a factory function also does not need to be a non-static member.
0
 

Author Comment

by:fungi8210
Comment Utility
i just added static to it, now i am getting these errors
error LNK2001: unresolved external symbol "public: static class CUnknown * __cdecl CFPSFilter::CreateInstance(struct IUnknown *,long *)" (?CreateInstance@CFPSFilter@@SAPAVCUnknown@@PAUIUnknown@@PAJ@Z)

error LNK2001: unresolved external symbol CLSID_FPSFilter
1>Windows Mobile 6 Professional SDK (ARMV4I)\Debug/MyFilter.dll : fatal error LNK1120: 2 unresolved externals

just incase you are wondering, i'm trying to write a directshow filter, and CLSID_FPSFilter is my classid defined at the top

// {033104B2-2AA9-4476-992E-AB504F9F1F1E}
DEFINE_GUID(CLSID_FPSFilter,
0x33104b2, 0x2aa9, 0x4476, 0x99, 0x2e, 0xab, 0x50, 0x4f, 0x9f, 0x1f, 0x1e);

btw, you answered my question correctly, you'll get the points, just wondering if you know how to solve my second problem, thanks!
0
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LVL 86

Expert Comment

by:jkr
Comment Utility
You'll need to provide an implementation also, e.g.

static CUnknown * WINAPI CMYFilter::CreateInstance(LPUNKNOWN pUnk, HRESULT *pHr) {

  CMYFilter* p = NULL;

  // other code to create a 'CMYFilter' here...

  return p;
}

0
 
LVL 86

Expert Comment

by:jkr
Comment Utility
Sorry, that should have been

CUnknown * WINAPI CMYFilter::CreateInstance(LPUNKNOWN pUnk, HRESULT *pHr) {

  CMYFilter* p = NULL;

  // other code to create a 'CMYFilter' here...

  return (CUnknown*) p;
}
0
 

Author Comment

by:fungi8210
Comment Utility
thanks, one last error

CMYFilter.obj : error LNK2001: unresolved external symbol CLSID_CMYFilter
1>Windows Mobile 6 Professional SDK (ARMV4I)\Debug/MyFilter.dll : fatal error LNK1120: 1 unresolved externals

its weird because i defined CLSID_CMYFilter at the top


// {033104B2-2AA9-4476-992E-AB504F9F1F1E}
DEFINE_GUID(CLSID_CMYFilter,
0x33104b2, 0x2aa9, 0x4476, 0x99, 0x2e, 0xab, 0x50, 0x4f, 0x9f, 0x1f, 0x1e);

0
 
LVL 86

Expert Comment

by:jkr
Comment Utility
>>its weird because i defined CLSID_CMYFilter at the top

Yes, but for that to be expanded to also an implementation (not just a declaration) you'll need to

#define INITGUID

before

#include <objbase.h>
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