since createTrackbar() wants the address of a static function, you need some redirection, pass the 'this' pointer as arg, and later resolve it to your class instance, so you can call a class-member function:

class Capture
{
    String name;
    VideoCapture cap;
    int trackValue;

    static void onChange(int v, void *ptr)
    {
        // resolve 'this':
        Capture *that = (Capture*)ptr;
        that->realTrack(v);
    }

    void realTrack(int v)
    {
        // do something with class members or v;
    }
public:
    Capture(const String &name, int id)
        : name(name)
        , cap(id)
        , trackValue(19)
    {
        namedWindow(name);
        createTrackbar("track1",name,&trackValue,100,onChange,this); // add 'this' as arg here.
    }

    bool nextFrame()
    {
        Mat frame;
        if ( ! cap.read(frame) )
            return false;
        // do something with frame
        putText(frame,format("%d",trackValue),Point(90,90),FONT_HERSHEY_PLAIN,2,Scalar(0,200,0));
        imshow(name, frame);
        if ( waitKey(10)==27 )
            return false;
        return true;
    }
};

int main()
{
    Capture cap("cap1",0);
    while( 1 )
    {
        if (! cap.nextFrame())
            break;
    }
    return 0;
}