Want a static member function to call a member variable of the same class

Question

headerfile.h

class A
{
  cv::Mat depthimagemouse;
  std::string m_winname;

public:

  A(const std::string &winname, const cv::Mat depth_clean);
  static void onMouse( int evt, int x, int y, int flags, void* param );
};

cppfile.cpp

A::A(const std::string &winname, const cv::Mat depth_clean)
    : m_winname(winname), depthimagemouse(depth_clean)
{
//something..
}

void A::onMouse( int event, int x, int y, int flags, void* param )
{
//Here I want to use depthimagemouse member variable (and other members..)
}

My question is how can I use depthimagemouse variable in onMouse method?

Answer 1

I'd be surprised if the library didn't explain this somewhere in its documentation, but anyway. This is standard procedure when you're using callbacks that don't support member functions, but you still need a way to access member data. So, you do the following:

• Pass a reference to the instance as your user data pointer param (or a member thereof) when registering the callback.
• Cast this back to the concrete type to access its members. A class static function has full access to all members of its class, via a provided instance.

So, you can do it this way:

auto &that = *static_cast<A *>(param); // <= C++03: use A &that
// Cast to const A if you want, or use a pointer, or etc.
std::cout << that.depthimagemouse << std::endl;

Or it's often nicer syntactically to immediately despatch to a member function and let it do everything:

static_cast<A *>(param)->doRestOfStuff(evt, x, y, flags);
// Include a const in the cast if the method is const

Or anywhere in between.

Answer 2

depthimagemouse is an instance member, means that each A instance (object if you prefer) has its own depthimagemouse . Your onMouse method is a static one, means that it is not related to any particular given instance but to all instances, and thus it's nonsense to think about accessing depthimagemouse without specifying which instance you are interested in.

Without any more information about onMouse and your model it is hard to tell you what to do. May be param can be used to specify an instance of A that the static method will receive? In that case, a cast can be used to get the instance back inside the method: A *anInstance = static_cast<A *>(param); with which you can then play: anInstance->depthimagemouse (see that we are talking about the depthimagemouse of a given instance?), etc.