using c++11 lambdas with opencv 3 trackbar callbacks

opencv-trackbar-lambda.cpp

#include <functional>

using TrackbarAction = std::function<void(int)>;

cv::namedWindow("win");

cv::TrackbarCallback trackbarCallback = [] (int pos, void* userdata) {
  (*(TrackbarAction*)userdata)(pos);
};

TrackbarAction action1 = [&](int pos) {
  std::cout << "trackbar 1 action" << std::endl;
};
cv::createTrackbar("trackbar 1", "win", &pos, posMax, trackbarCallback, (void*)&action1);

TrackbarAction action2 = [&](int pos) {
  std::cout << "trackbar 2 action" << std::endl;
};

cv::createTrackbar("trackbar 2", "win", &pos, posMax, trackbarCallback, (void*)&action2);

It doesn't work if lambda takes some context from locally visible namespace e.g.:

Mat src;
cv::TrackbarCallback doWholeMagicWrapper = [&src](int, void*){ do_something_here_with_src(src) };

gives compilation error:

error: cannot convert ‘main(int, char**)::<lambda(int, void*)>’ to ‘cv::TrackbarCallback {aka void (*)(int, void*)}’ in initialization
     cv::TrackbarCallback doWholeMagicWrapper = [&src](int, void*){};

Do you have any idea hot to go with it?

It doesn't work if lambda takes some context from locally visible namespace e.g.:

Mat src;
cv::TrackbarCallback doWholeMagicWrapper = [&src](int, void*){ do_something_here_with_src(src) };

gives compilation error:

error: cannot convert ‘main(int, char**)::<lambda(int, void*)>’ to ‘cv::TrackbarCallback {aka void (*)(int, void*)}’ in initialization
     cv::TrackbarCallback doWholeMagicWrapper = [&src](int, void*){};

Do you have any idea hot to go with it?

Use std::tuple and pass the data as void pointer. Example usage