Another keyboard event question in C++

C++ coding using Panda3D
1

Hi all (again),

I’ve been busy translating the Roaming Ralph tutorial to C++. It’s almost done – Just the animation part.
But, I’m having some trouble with the keypresses too. Usually, it works fine:

void exit_event(const Event * theEvent, void * data) {
  exit(0);
}
//...
World::World(WindowFramework *window) {
  //...
  framework_ptr=window->get_panda_framework();
  //...
  framework_ptr->define_key("escape", "Quit", exit_event, 0);
  //...
}

Though, once I want an event to a function within my class World, It won’t work anymore:

World::World(WindowFramework *window) {
  //...
  pair<string, bool> data=("left", true);
  framework_ptr->define_key("arrow_left", "Rotate Ralph Left", set_key, (void *) &data);
  //...
}
World::set_key(const Event * theEvent, void * data) {
  //...
}

That throws an error because there is no overloaded function for define_key. If I make set_key a global function instead of a subfunction of the class World it works.

Does anybody have an idea how to correct this? Thanks in advance.

2

The problem is the hidden “this” pointer to set_key(), which makes it no longer match the function prototype expected by define_key(). To avoid this, you have to declare set_key() to be a static method. (You can use the data pointer to pass in the this pointer again.)

David

3

Oh, right, sorry, forgot about that. I’ll try out your suggestion.

4

OK, so this is very annoying. Since I can’t access non-static members from within a static function, I have to define all variables I want to use from within that function globally.
Isn’t there a way to tell define_key that it takes another argument, the this pointer? Or perhaps a better way to do things?

5

I usually write a static function that extracts the this parameter from its data parameter, and then uses that this pointer to call a non-static version of itself.

David

6

Excellent solution! Thanks a lot.