How can I pass a C++ member function to a C API as a parameter

Question

In my C++ program, I need to call this c API:

GConn* gnet_conn_new (const gchar *hostname,
                      gint port,
                      GConnFunc func);

where GConnFunc is defined as:

void (*GConnFunc) (GConn *conn);

My question is if I have a C++ class and have a member function like:

Class A {
 public:
   A();
   void my_func (GConn* conn);
}

In my A::A() Constructor, how can I pass this->myfunc to gnet_conn_new as the GConnFunc parameter?

Thank you.

Answer 1

Most APIs provide a pointer-sized 'user data' argument, which would look like this:

GConn* gnet_conn_new (const gchar *hostname, 
                  gint port, 
                  GConnFunc func,
                  void* user_data); 

This would allow you to pass an instance in user_data and have your C function forward to the member function like this:

void my_func(GConn *conn, void* user_data)
{
    ((MyClass*)user_data)->MyMemberFunc(conn);
}

Perhaps your API has an alternative or similar 'user data' parameter somewhere. Otherwise, you can't really do it without globals or statics.