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the trackbar callback expects a global, static callback function (it has to save the function address). if you want to use this in a class context, you need some indirection ("dispatch" from a static handler to your desired class member function):

struct Foo {
    int value;    

    // static  function
    static void callback(int pos, void *userptr) {
        Foo *foo = (Foo*)usrptr; // cast user data back to "this"
        foo->real_callback(pos);
    }

    // class member function
    void real_callback(int p) {
         // do something.
         // you can use Foo's "this" ptr here.
         this->value = p;
    }

};


...
Foo foo;

// now pass Foo's *static* callback, and it's address:
cv::createTrackbar(win_name, bar_name, foo.value, 100, Foo::callback, (void*)(&foo));

the trackbar callback expects a global, static callback function (it has to save the function address). if you want to use this in a class context, you need some indirection ("dispatch" from a static handler to your desired class member function):

struct Foo {
    int value;    

    // static  function
    static void callback(int pos, void *userptr) {
        Foo *foo = (Foo*)usrptr; // cast user data back to "this"
        foo->real_callback(pos);
    }

    // class member function
    void real_callback(int p) {
         // do something.
         // you can use Foo's "this" ptr here.
         this->value = p;
    }

};


...
Foo foo;

// now pass Foo's *static* callback, and it's address:
cv::createTrackbar(win_name, bar_name, foo.value, &foo.value, 100, Foo::callback, (void*)(&foo));