// file: main.cpp
#include "app.h"

int main() {
  App app(400, 300, "APP");
  return Fl::run();
}

// file: app.h
#include <FL/Fl_Window.H>

class AppImpl {
public:
  AppImpl(int w, int h, const char *title) {
    this->m_win = new Fl_Window(w, h, title);
    this->m_win->show();
  }
  ~AppImpl() {
    if (this->m_win) {
      delete this->m_win;
      this->m_win = nullptr;
    }
  }
private:
  Fl_Window *m_win;
};

class App {
public:
  App(int w, int h, const char *title) { m_impl = new AppImpl(w, h, title); }
  ~App(){};

private:
  AppImpl *m_impl;
};