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POAccessing variable outside scope of a callback c++
 

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  1. POAccessing variable outside scope of a callback c++
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    <p>I have been beating my head around this issue of static versus non-static, callback functions, function pointers, etc... My goal is to access data of a struct outside the scope of my callback interface. I am trying to do this within my class called TextDetect. I thought I was on track when I asked this question: <a href="https://stackoverflow.com/questions/11513071/avoiding-a-static-member-function-in-c-when-using-a-callback-interface-from-c">Avoiding a static member function in c++ when using a callback interface from C</a> However, I still can't access the data without losing scope over the data that I am most interested. At runtime, I get "Access violation reading location ..." I'll point it out below where it fails. I implemented the answer to my previous question as the following class, shown entirely (Note: vtrInitialize is part of a 3rd party api code <strong>int vtrInitialize(const char *inifile, vtrCallback cb, void *calldata);</strong>):</p> <pre><code> class TextDetect { const char * inifile; vtrImage *vtrimage; int framecount; public: TextDetect(); ~TextDetect(); void vtrCB(vtrTextTrack *track); static void vtrCB_thunk(vtrTextTrack *track, void *calldata); int vtrTest(cv::Mat); bool DrawBox(cv::Mat&amp;); vtrTextTrack *texttrack; }; TextDetect::TextDetect() : inifile("vtr.ini") { if (vtrInitialize(inifile, vtrCB_thunk, static_cast&lt;void *&gt;(this) ) == -1) std::cout &lt;&lt; "Error: Failure to initialize" &lt;&lt; std::endl; vtrimage = new vtrImage; } int TextDetect::vtrTest(cv::Mat imagetest) { /*store image data in an image structure*/ } void TextDetect::vtrCB(vtrTextTrack *track) { /*send data to command line from callback */ </code></pre> <p>I've tried copying the data I need a variety of ways and nothing works (this code is a continuation from above):</p> <pre><code> //texttrack = track; //texttrack = new vtrTextTrack (*track); memcpy(texttrack,track,sizeof(*track)); //vtrTextTrackFree(track); } void TextDetect::vtrCB_thunk(vtrTextTrack *track, void *calldata) { static_cast&lt;TextDetect *&gt;(calldata)-&gt;vtrCB(track); } </code></pre> <p>This is the member function were I want the data to be used. Texttrack is public member so I might need it outside my class as well (this code is a continuation from above):</p> <pre><code> bool TextDetect::DrawBox(cv::Mat&amp; tobeboxed) { </code></pre> <p>And I get the access violation error at runtime here at this line of code (this code is a continuation from above):</p> <pre><code> if (texttrack-&gt;best-&gt;ocrconf &gt; 90) { /*do some more stuff*/ } } </code></pre>
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    1. USuser1530101
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    1. COhonestly, is really that much code necessary to depict your problem?
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      1. USTorsten Robitzki
    2. COThe problem you have in the form it is written is difficult to understand quickly; perhaps simplifying it will prompt more people to answer.
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      1. USSeth Carnegie
    3. COWell, your `memcpy` is certainly an error: it cannot possibly copy all of `track`, as you obviously have pointers in the `track`, which will be copied, but not what they point to. I can easily imagine, that the `track` data do not exist outside the callback.
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      1. USfork0
 

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