Question

I am using a C library which has callback functions such as:

int DownloadStream(LPCTSTR  host,DWORD channelNumber,NewImage pNewImage);
typedef int  (*NewImage)(BYTE *pData, int nLen,void *returnHandle);

In C i can make call like this:

int  CallDownloadStream(LPCTSTR  host,DWORD channelNumber)
{

  int hr = DownloadStream(host,channelNumber,OnNewImage);

  return hr;

}


int OnNewImage(BYTE *pData, int nLen,void *returnHandle)
{

   // able to get data

}

What i want is call those function as a member of C++ class such as:

class MyClass
{

public:


   int  CallDownloadStream(LPCTSTR  host,DWORD channelNumber)
   {

       int hr = DownloadStream(host,channelNumber,OnNewImag);

       return hr;
   }

   int OnNewImage(BYTE *pData, int nLen,void *returnHandle)
   {

   // able to get data

   }
}

I can not able to compile it. Is it possible to use those callback function in that way? If so how can do this?

PS: I have no control over original C style callback functions.

Answer 1

Cannot be done. If the C function does not provide for a user-supplied argument pointer, which is idiomatic in C, then you cannot do this.

Edit: This isn’t strictly true. You can use a static variable, you can use thread-local storage, or you can JIT a function. However, those are not general-case solutions.