将C++类铸造到另一个类中以创建OpenCV轨迹条句柄

Question

为了不干扰全局变量和函数，我想在OpenCV中使用类的函数作为轨迹条句柄的函数。下面的代码说明了这个想法：

void cam::set_Trackbarhandler(int i, void *func)
{
    /* This function might be called whenever a trackbar position is changed */
}
void cam::create_Trackbars(void)
{
    /**
     * create trackbars and insert them into main window.
     * 3 parameters are:
     * the address of the variable that is changing when the trackbar is moved,
     * the maximum value the trackbar can move,
     * and the function that is called whenever the trackbar is moved
     */
    const string trck_name = "Exposure";
    char wnd_name[128];
    get_Mainwindowname(wnd_name, sizeof(wnd_name));

    createTrackbar(trck_name, //Name of the trackbar
                    wnd_name, //Name of the parent window
                    &setting, //value that's changed
                    (int)out_max, //maximum value
                    this->set_Trackbarhandler); //pointer to the function that's called
}

我希望概述一下。我在编译时遇到的错误

error: cannot convert 'cam::set_Trackbarhandler' from type 'void (cam::)(int, void*)' to type 'cv::TrackbarCallback {aka void (*)(int, void*)}'|

有没有一种方法可以将void (cam::)(int, void*)转换为简单的void (*)(int, void*)，或者我必须使用全局函数，即

void set_Trackbarhandler(int i, void *func)

如果必须这样做，我的最后手段是使用void指针（请参阅http://docs.opencv.org/modules/highgui/doc/user_interface.html)并将指向该类的指针作为发送回

    createTrackbar(trck_name,
                    wnd_name,
                    &setting,
                    (int)out_max,
                    set_Trackbarhandler, //now a global function
                    this);

我想。在set_Trackbarhandler函数中，我会制作一个类似的投射

cam *ptr = static_cast<cam *>(func);

不过听起来有点复杂。

Answer 1

好吧。你需要一些间接，但也没那么糟糕。。。

class cam
{
public:
    void myhandler(int value)
    {
       // real work here, can use 'this'
    }
    static void onTrack(int value, void* ptr)
    {
        cam* c = (cam*)(ptr);
        c->myhandler(value);
    }
};
createTrackbar(trck_name,
                    wnd_name,
                    &setting,
                    (int)out_max,
                    cam::onTrack, //now a static member
                    this);