callfunction in class

[Guest_imported]

how can I make a memberfunction to be called as a callback.<br>I am writing a imageprocessing-program and grab the image from a webcam. I want to make the framecallbackfunction a memberfunction.

 

[Karl Blessing]

This was taken from MSDN online(msdn.microsoft.com) <br><br><FONT FACE=monospace><br>Callback Function Example<br>This example uses a callback function (fnReceiveCallback is not NULL) to read the first message in a queue. In this case, the message is read by MQReceiveMessage and the returned message properties (and error codes) are handled by the registered callback function.<br><br>Note&nbsp;&nbsp;Once registered, a callback function cannot be unregistered. Closing the queue, closes the callback function as well.<br><br>When using a callback function, it is very important to allocate the memory for the MQMSGPROPS structure on the heap. Not on the stack. In addition, always free MQMSGPROPS within the callback function<br><br>When multiple asynchronous MQReceiveMessage calls are outstanding, several callbacks are registered; upon arrival of a message, the first registered callback is called. Internally, MSMQ uses the WaitForMultipleObjects API which is limited to 64 objects per process.<br><br>To read a single message using a callback function <br><br>Write the callback function. <br>Call MQOpenQueue to open the queue with receive or peek access. If the queue is opened with receive access, the application can still peek at the messages in the queue. <br>Specify the message properties to be retrieved, retrieving only those properties that are needed. <br>Initialize the MQMSGPROPS structure on the heap. <br>Call MQReceiveMessage to read the message from the queue, making sure to free the MQMSGPROPS structure from within the callback function. <br>Call MQCloseQueue to close the queue, freeing all memory allocated for any pending MQReceiveMessage calls. Closing the queue, closes the callback function as well. <br>Code Example<br>The following example reads the first message of the queue using a callback function.<br><br>//////////////////////////////<br>// Receive callback function.<br>/////////////////////////////<br>&nbsp;<br>void APIENTRY ReceiveCallbackRoutine(<br>&nbsp;&nbsp;&nbsp;HRESULT hr,<br>&nbsp;&nbsp;&nbsp;QUEUEHANDLE hSource,<br>&nbsp;&nbsp;&nbsp;DWORD dwTimeout,<br>&nbsp;&nbsp;&nbsp;DWORD dwAction,<br>&nbsp;&nbsp;&nbsp;MQMSGPROPS* pMessageProps,<br>&nbsp;&nbsp;&nbsp;LPOVERLAPPED lpOverlapped,<br>&nbsp;&nbsp;&nbsp;HANDLE hCursor<br>&nbsp;&nbsp<br>&nbsp;{<br>&nbsp;&nbsp;if (FAILED(hr))<br>&nbsp;&nbsp;&nbsp;{<br>&nbsp;&nbsp;&nbsp;&nbsp;// Error handler for Callback routine.<br>&nbsp;&nbsp;&nbsp;}<br>&nbsp;&nbsp;else<br>&nbsp;&nbsp;{<br>&nbsp;&nbsp;&nbsp;// Process message.<br>&nbsp;&nbsp;}<br>&nbsp;}<br><br>//////////////<br>// Open Queue<br>//////////////<br>&nbsp;<br>HRESULT hr;<br>QUEUEHANDLE hQueue;<br>&nbsp;<br>hr = MQOpenQueue(<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;szwFormatNameBuffer,&nbsp;&nbsp;&nbsp;&nbsp;// Format Name of the queue to be opened<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MQ_RECEIVE_ACCESS,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Access rights to the Queue<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// No receive Exclusive<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&hQueue&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// OUT: handle to the opened Queue<br>);<br>&nbsp;<br>if (FAILED(hr))<br>{<br>&nbsp;// Error handler for MQOpenQueue.<br>}<br>&nbsp;<br>&nbsp;<br>MQMSGPROPS * pmsgprops;<br>MQPROPVARIANT *paVariant;<br>MSGPROPID * paPropId;<br>DWORD dwcPropId = 0;<br>&nbsp;<br>//<br>//&nbsp;&nbsp;The output parameters to an asynchronous call to MQReceiveMessage <br>//&nbsp;&nbsp;should be kept intact until the operation completes, you should <br>//&nbsp;&nbsp;not free or reuse them until the operation is complete.<br>//<br>pmsgprops = new MQMSGPROPS;<br>paVariant = new MQPROPVARIANT[ 10];<br>paPropId = new MSGPROPID[ 10];<br>&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;<br>//////////////////////////////////////////////////<br>// Prepare the message properties to be retrieved.<br>/////////////////////////////////////////////////<br>&nbsp;<br>&nbsp;<br>// Set the PROPID_M_BODY property.<br>paPropId[dwcPropId] = PROPID_M_BODY;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//PropId<br>paVariant[dwcPropId].vt = VT_VECTOR¦VT_UI1;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//Type<br>paVariant[dwcPropId].caub.cElems = MSG_BODY_LEN ;&nbsp;&nbsp;&nbsp;&nbsp;//Value<br>paVariant[dwcPropId].caub.pElems = new unsigned char[ MSG_BODY_LEN];<br>&nbsp;<br>&nbsp;dwcPropId++;<br>&nbsp;<br>////////////////////////////////<br>// Initialize the MQMSGPROPS structure<br>///////////////////////////////<br>pmsgprops-&gt;cProp = dwcPropId;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//Number of properties<br>pmsgprops-&gt;aPropID = paPropId;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//Ids of properties<br>pmsgprops-&gt;aPropVar = paVariant;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//Values of properties<br>pmsgProps-&gt;aStatus&nbsp;&nbsp;= NULL;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//No Error report<br>&nbsp;<br>&nbsp;<br>///////////////////////////////////////////////<br>//&nbsp;&nbsp;Receive the message using callback function<br>//&nbsp;&nbsp;ReceiveCallbackRoutine.<br>///////////////////////////////////////////////<br>&nbsp;<br>hr = MQReceiveMessage(<br>&nbsp;&nbsp;&nbsp;&nbsp;hQueue,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Handle to the Queue<br>&nbsp;&nbsp;&nbsp;&nbsp;5 * 60 * 1000,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Max time (msec) to wait<br>&nbsp;&nbsp;&nbsp;&nbsp;MQ_ACTION_RECEIVE,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Action<br>&nbsp;&nbsp;&nbsp;&nbsp;pmsgprops,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Properties to retrieve<br>&nbsp;&nbsp;&nbsp;&nbsp;NULL,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// No OVERLAPPED structure<br>&nbsp;&nbsp;&nbsp;&nbsp;ReceiveCallbackRoutine,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Callback function<br>&nbsp;&nbsp;&nbsp;&nbsp;NULL,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// No Cursor<br>&nbsp;&nbsp;&nbsp;&nbsp;NULL&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// No transaction<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;<br>if (FAILED(hr))<br>&nbsp;&nbsp;&nbsp;{<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//&nbsp;&nbsp;Error handler for MQReceiveMessage.<br>&nbsp;&nbsp;&nbsp;&nbsp;}<br><br>Built on Wednesday, January 05, 2000<br></font><br> <p>Karl<br><a href=mailto:kb244@kb244.com>kb244@kb244.com</a><br><a href=

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</a><br>Experienced in : C++(both VC++ and Borland),VB1(dos) thru VB6, Delphi 3 pro, HTML, Visual InterDev 6(ASP(WebProgramming/Vbscript)<br>

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[davidewan]

You can't make a regular member function a callback because you can't pass a 'this' (all regular member functions have a hidden first argument 'this'). You can get around this by making the member function static. Static member functions have no 'this'.