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.NET Forum / Languages / Managed C++ / August 2006Old style API function callback conversion to managed C++
I am wrapping a digital camera API using Managed C++ VS .NET 2003). I have this function that called as bellow in the API sample. err = PR_RC_StartViewFinder( m_hCamera, //line 1 (prContext)this, //line 2 (prViewFinderCB*)&ViewFinderCallBackFun ); //line 3 prContext is actually a typedef for unsigned long. ViewFinderCallBackFun is a callback function. There is two error that I get when i tried above code unchanged: Line 2-> error C2440: 'type cast' : cannot convert from 'CameraSDK::Camera __gc *const ' to 'prContext'. Cannot cast a __gc pointer to an integral type Line 3 -> error C2276: '&' : illegal operation on bound member function expression. Thanks in advance! > I am wrapping a digital camera API using Managed C++ VS .NET 2003). I > have this function that called as bellow in the API sample. [quoted text clipped - 14 lines] > > Thanks in advance! It seems that line 3 error is because I don't pun static in ViewFinderCallBackFun declaration. So now it is Line 2 error that is troubling me. Is it possible to pass this to unmanaged API function? TIA > > I am wrapping a digital camera API using Managed C++ VS .NET 2003). I > > have this function that called as bellow in the API sample. [quoted text clipped - 20 lines] > So now it is Line 2 error that is troubling me. Is it possible to pass > this to unmanaged API function? Your code as it is now tries to pass a managed pointer as an unmanaged pointer. You could pin the pointer (__pin) to solve that. Check the documentation for __pin. There are a couple of examples there that show you how to use it.  SignatureKind regards, Bruno. bruno_nos_pam_van_dooren@hotmail.com Remove only "_nos_pam" Bruno wrote: > > > I am wrapping a digital camera API using Managed C++ VS .NET 2003). I > > > have this function that called as bellow in the API sample. [quoted text clipped - 32 lines] > bruno_nos_pam_van_dooren@hotmail.com > Remove only "_nos_pam" Thanks for the pointer :D I try this and somehow it doesnt give me compile error Camera __pin *This = this; prContext pCon = (prContext) This; CheckAPI(PR_RC_StartViewFinder( m_hCamera, pCon, (prViewFinderCB*)&ViewFinderCallBackFun)); It is not working yet, will do further testing. In the mean time, appreciate any feedback. Thanks! > Bruno wrote: > > > > I am wrapping a digital camera API using Managed C++ VS .NET 2003). I [quoted text clipped - 50 lines] > > Thanks! It doesnt work, PR_RC_StartViewFinder return error. I think this is because __pin *This scope is short. The scope end right after PR_RC_StartViewFinder is called. According to the MSDN documentation --------------------- A pinned object is pinned only while a pinning pointer points to it. It is no longer pinned when its pinning pointer goes out of scope, or is set to 0 in the program. After that, any unmanaged pointers that remain pointing to that object must not be dereferenced. An unpinned item can be moved in the heap by the garbage collector. Any __nogc pointers that still point to it will not be updated, and dereferencing one of them could raise an unrecoverable exception. ---------------------- I still need help to get arround this. Any help is appreciated! >> I try this and somehow it doesnt give me compile error >> [quoted text clipped - 24 lines] > > I still need help to get arround this. Any help is appreciated! Well, Is there a reason why you want to pass a pointer of a managed object to the SDK? Your design would be a lot simpler if you would only use unmanaged C++ to work together with the SDK. Perhaps a good solution would be to write a very thin unmanaged wrapper class that abstracts the SDK (C style functions with callback) from your managed code. And then you let your managed code interact only with your wrapper class in a way that does not require it to supply callback functions. This would have almost 0 performance impact, while greatly simplifying your design. SignatureKind regards, Bruno van Dooren bruno_nos_pam_van_dooren@hotmail.com Remove only "_nos_pam" Hello, >I am wrapping a digital camera API using Managed C++ VS .NET 2003). I > have this function that called as bellow in the API sample. [quoted text clipped - 12 lines] > Line 3 -> error C2276: '&' : illegal operation on bound member function > expression. Why do you think it's appropriate to pass managed pointer into the native function? Do you have an example for this API? P.S. There is nothing old in this style of callback API. SignatureVladimir Nesterovsky > Hello, > [quoted text clipped - 17 lines] > Why do you think it's appropriate to pass managed pointer into the native > function? I was at that time looking for quick fix to my problem and experiment here and there on what I can do with mix managed and unmanaged class. It appears that __pin allow me to pass "this" pointer to unmanaged class. It does work. however please read my conclussion at the end of this post. > Do you have an example for this API? The example that I get for this API is purely c++ (not managed extension sample). > P.S. There is nothing old in this style of callback API. I am referring to it with delegate callback style in mind. But you are right that there is nothing old about it since delegate mainly applies to .NET or managed c++. Well I can be wrong about this too. > -- > Vladimir Nesterovsky At the end of the experiment process I have decided to follow Bruno van Dooren advise to use unmanaged C++ to interact with the SDK. By doing this there is no need to pin "this" pointer. I have choosen this as I have some doubt regarding pinning "this" pointer and passing it to callback function that will run through the application/dll runtime. My initial __pin "this" try is successful but I suspect that this is a time bomb waiting to explode :D. Thanks all and best regards!
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