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POFunction pointer to class member function problems
 

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    <p>First of all I have to admit that my programming skills are pretty limited and I took over a (really small) existing C++ OOP project where I try to push my own stuff in. Unfortunately I'm experiencing a problem which goes beyond my knowledge and I hope to find some help here. I'm working with a third party library (which cannot be changed) for grabbing images from a camera and will use some placeholder names here. </p> <p>The third party library has a function "ThirdPartyGrab" to start a continuous live grab and takes a pointer to a function which will be called every time a new frame arrives. So in a normal C application it goes like this:</p> <pre><code>ThirdPartyGrab (HookFunction); </code></pre> <p>"HookFunction" needs to be declared as: </p> <pre><code>long _stdcall HookFunction (long, long, void*); </code></pre> <p>or "BUF_HOOK_FUNCTION_PTR" which is declared as</p> <pre><code>typedef long (_stdcall *HOOK_FUNCTION_PTR) (long, long, void*); </code></pre> <p>Now I have a C++ application and a class "MyFrameGrabber" which should encapsulate everything I do. So I put in the hook function as a private member like this:</p> <pre><code>long _stdcall HookFunction (long, long, void*); </code></pre> <p>Also there is a public void function "StartGrab" in my class which should start the Grab. Inside I try to call:</p> <pre><code>ThirdPartyGrab (..., HookFunction, ...); </code></pre> <p>which (not surprisingly) fails. It says that the function call to MyFrameGrabber::HookFunction misses the argument list and I should try to use &amp;MyFrameGrabber::HookFunction to create a pointer instead. However passing "&amp;MyFrameGrabber::HookFunction" instead results in another error that this cannot be converted to BUF_HOOK_FUNCTION_PTR. </p> <p>After reading through the <a href="http://www.parashift.com/c++-faq-lite/pointers-to-members.html" rel="nofollow noreferrer">C++ FAQ function pointers</a> I think I understand the problem but can't make up a solution. I tried to make the hook function static but this also results in a conversion error. I also thought of putting the hook function outside of the class but I need to use class functions inside the hook function. Is there another way or do I need to change my whole concept? </p> <p>EDIT 14.01.08: I tested the singleton workaround since I cannot change the third party library and the void pointer is only for data that is used inside the hook function. Unfortunately it didn't worked out of the box like I hoped.... I don't know if the static function needs to be in a separate class so I put it in my "MyFrameGrabber" class:</p> <pre><code>static MyFrameGrabber&amp; instance() { static MyFrameGrabber _instance; return _instance; } long Hook(long, long, void*); // Implementation is in a separate cpp file </code></pre> <p>In my cpp file I have the call_hook function:</p> <pre><code>long MFTYPE call_hook(long x, MIL_ID y, void MPTYPE *z) { return MyFrameGrabber::instance().Hook(x,y,z); } void MyFrameGrabber::grab () { ThirdPartyGrab(..., call_hook, ...); } </code></pre> <p>But this gives me an error in <code>static MatroxFrameGrabber _instance;</code> that no matching standard constructor is found. That's correct because my MyFrameGrabber constructor looks like this:</p> <pre><code>MyFrameGrabber (void* x, const std::string &amp;y, int z, std::string &amp;zz); </code></pre> <p>I tried to put in an empty constructor <code>MyFrameGrabber();</code> but this results in a linker error. Should I pass empty parameters to the MyFrameGrabber constructor in the singleton? Or do I need to have a separate Hook Class and if yes how could I access MyFrameGrabber functions? Thanks in advance. </p> <p>SECOND EDIT 15.01.08: I applied the changes and it compiles and links now. Unfortunately I cannot test this at runtime yet because it's a DLL and I have no Debug Caller Exe yet and there are other problems during initialization etc. I will mark the post as answer because I'm sure this is the right way to do this.</p>
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