Getting used to object oriented programming?

I think I am now having less difficulty than before.

/* main.cpp */
#include "MyWindow.h"
#include "MyArea.h"
#include "MyButton.h"
#include <gtkmm.h>

int main(int argc, char** argv)
{
   Glib::RefPtr<Gtk::Application> app = Gtk::Application::create(argc, argv, "org.gtkmm.example");

   MyWindow win;

   win.set_title("Object Oriented?");
   win.set_default_size(400, 400);
   win.show_all_children();

   return app->run(win);
}

/* MyWindow.h */
#ifndef MYWINDOW_H_
#define MYWINDOW_H_
#include "MyArea.h"
#include "MyButton.h"
#include <gtkmm.h>

class MyWindow : public Gtk::Window {
public:
	MyWindow();
	virtual ~MyWindow();
protected:
	Gtk::Box m_box;
	MyArea   m_area;
	MyButton m_buttons;
};

#endif /* MYWINDOW_H_ */

/* MyWindow.cpp */
#include "MyWindow.h"
#include <gtkmm.h>

MyWindow::MyWindow() : m_box(Gtk::ORIENTATION_VERTICAL) {
	add(m_box);
	m_box.pack_start(m_area);
	m_box.pack_start(m_buttons, false, true);
}

MyWindow::~MyWindow() {
}

/* MyArea.h */
#ifndef MYAREA_H_
#define MYAREA_H_

#include <gtkmm.h>

class MyArea : public Gtk::DrawingArea
{
public:
  MyArea();
  virtual ~MyArea();

protected:
  virtual bool on_draw(const Cairo::RefPtr<Cairo::Context>& cr);
 };

#endif /* MYAREA_H_ */

/* MyArea.cpp */
#include "MyArea.h"
#include <gtkmm.h>
#include <cairomm/context.h>
#include <complex>
using namespace std;

double x_min =  -1.5;
double x_max =   0.5;
double y_min =  -1.0;
double y_max =   1.0;

MyArea::MyArea()
{
}

MyArea::~MyArea()
{
}

bool MyArea::on_draw(const Cairo::RefPtr<Cairo::Context>& cr)
{
  Gtk::Allocation allocation = get_allocation();
  const int width  = allocation.get_width();
  const int height = allocation.get_height();

  cr->set_line_width(1.0);

  double large =  100.0;
  int    kmax  = 50;

  for(int i=0;i<width;i++) {
	  for(int j=0;j<height;j++) {
		  double x=x_min+(x_max-x_min)*(double)i/(double)width;
		  double y=y_min+(y_max-y_min)*(double)j/(double)height;
		  complex<double> z=0.0;
		  complex<double> w=complex<double>(x,y);
		  int count=0;
		  for(int k=0;k<kmax;k++) {
			  z=z*z+w;
			  if(abs(z)>large) {
				  count = k;
				  break;
			  }
		  }
		  if(count) {
			  cr->set_source_rgba(0.9,(count%10)/10.0,0.3,1.0);
		  } else {
			  cr->set_source_rgba(0.0,0.2,0.7,1.0);
		  }
		  cr->rectangle((double)i, (double)j, 1.0, 1.0);
		  cr->stroke();
	  }
  }
  return true;
}

/* MyButton.h */
#ifndef MYBUTTON_H_
#define MYBUTTON_H_
#include <gtkmm.h>

class MyButton : public Gtk::Box {
public:
	MyButton();
	virtual ~MyButton();
protected:
	void on_button1_clicked();
	void on_button2_clicked();
	void on_button3_clicked();
	void on_button4_clicked();
	void on_button5_clicked();
	void on_button6_clicked();
	Gtk::Button m_button1;
	Gtk::Button m_button2;
	Gtk::Button m_button3;
	Gtk::Button m_button4;
	Gtk::Button m_button5;
	Gtk::Button m_button6;
};

#endif /* MYBUTTON_H_ */

/* MyButton.cpp */
#include "MyButton.h"

extern  double x_min;
extern  double x_max;
extern  double y_min;
extern  double y_max;

MyButton::MyButton() : m_button1("enlarge"), m_button2("shrink"), m_button3("left"), m_button4("right"), m_button5("up"), m_button6("down") {
	pack_start(m_button1);
	pack_start(m_button2);
	pack_start(m_button3);
	pack_start(m_button4);
	pack_start(m_button5);
	pack_start(m_button6);
	m_button1.signal_clicked().connect(sigc::mem_fun(*this, &MyButton::on_button1_clicked) );
	m_button2.signal_clicked().connect(sigc::mem_fun(*this, &MyButton::on_button2_clicked) );
	m_button3.signal_clicked().connect(sigc::mem_fun(*this, &MyButton::on_button3_clicked) );
	m_button4.signal_clicked().connect(sigc::mem_fun(*this, &MyButton::on_button4_clicked) );
	m_button5.signal_clicked().connect(sigc::mem_fun(*this, &MyButton::on_button5_clicked) );
	m_button6.signal_clicked().connect(sigc::mem_fun(*this, &MyButton::on_button6_clicked) );
}

MyButton::~MyButton() {
}

void MyButton::on_button1_clicked() {
	double xc = (x_min + x_max) / 2.0;
	double yc = (y_min + y_max) / 2.0;
	double xs = (x_max - xc) * 0.75;
	double ys = (y_max - yc) * 0.75;
	x_min = xc - xs;
	x_max = xc + xs;
	y_min = yc - ys;
	y_max = yc + ys;
	get_window()->invalidate(true);
}

void MyButton::on_button2_clicked() {
	double xc = (x_min + x_max) / 2.0;
	double yc = (y_min + y_max) / 2.0;
	double xs = (x_max - xc) / 0.75;
	double ys = (y_max - yc) / 0.75;
	x_min = xc - xs;
	x_max = xc + xs;
	y_min = yc - ys;
	y_max = yc + ys;
	get_window()->invalidate(true);
}

void MyButton::on_button3_clicked() {
	double xc = (x_min + x_max) / 2.0;
	double xs = (x_max - xc) * 0.75;
	x_min += xs;
	x_max += xs;
	get_window()->invalidate(true);
}

void MyButton::on_button4_clicked() {
	double xc = (x_min + x_max) / 2.0;
	double xs = (x_max - xc) * 0.75;
	x_min -= xs;
	x_max -= xs;
	get_window()->invalidate(true);
}

void MyButton::on_button5_clicked() {
	double yc = (y_min + y_max) / 2.0;
	double ys = (y_max - yc) * 0.75;
	y_min += ys;
	y_max += ys;
	get_window()->invalidate(true);
}

void MyButton::on_button6_clicked() {
	double yc = (y_min + y_max) / 2.0;
	double ys = (y_max - yc) * 0.75;
	y_min -= ys;
	y_max -= ys;
	get_window()->invalidate(true);
}