class SRadioButton : public Gtk::Window
{
public:
SRadioButton(string str = "Go to hell from ToggleButton")
{
SimpleRadioButton1 = new Gtk::RadioButton("Super Saiyan One");
SimpleRadioButton2 = new Gtk::RadioButton("Super Saiyan Two");
SimpleRadioButton3 = new Gtk::RadioButton("Super Saiyan Three");

Gtk::RadioButton::Group group = SimpleRadioButton1->get_group();
SimpleRadioButton2->set_group(group);
SimpleRadioButton3->set_group(group);
SimpleRadioButton4 = Gtk::manage(new Gtk::RadioButton(group,"Super Saiyan Four"));

set_title("My application");
set_border_width(10);

SimpleRadioButton1->set_active();
SimpleRadioButton1->signal_clicked().connect( sigc::mem_fun(*this,&SRadioButton:n_button_clicked) );
SimpleRadioButton2->signal_clicked().connect( sigc::mem_fun(*this,&SRadioButton:n_button_clicked) );
SimpleRadioButton3->signal_clicked().connect( sigc::mem_fun(*this,&SRadioButton:n_button_clicked) );
SimpleRadioButton4->signal_clicked().connect( sigc::mem_fun(*this,&SRadioButton:n_button_clicked) );

add(outer_box);
outer_box.pack_start(*SimpleRadioButton1);
outer_box.pack_start(*SimpleRadioButton2);
outer_box.pack_start(*SimpleRadioButton3);
outer_box.pack_start(*SimpleRadioButton4);
show_all_children();


}

virtual ~SRadioButton(){}

protected:
void on_button_clicked()
{
string str;
if(SimpleRadioButton1->get_active() == true)
{
str = SimpleRadioButton1->get_label() + " level reached";
}
else if(SimpleRadioButton2->get_active() == true)
{
str = SimpleRadioButton2->get_label() + " level reached";
}
else if(SimpleRadioButton3->get_active() == true)
{
str = SimpleRadioButton3->get_label() + " level reached";
}
else if(SimpleRadioButton4->get_active() == true)
{
str = SimpleRadioButton4->get_label() + " level reached";
}
cout<<str<<endl;
}

Gtk::RadioButton *SimpleRadioButton1,*SimpleRadioButton2,*SimpleRadioButton3,*SimpleRadioButton4;
Gtk::VBox outer_box;
};


Note:-Form has VBox widget containing these four radiobuttons only.

Problem:- When I click on a different button the message is printed twice.I actually want to select a radio button and want to print his label as the message only once.Please correct me if I am wrong.Thanks........

When you select a radio button, two events are generated in the following order:

1. The selected radio button is made active
2. Another radio button is made inactive

When each event is generated, your code is determining that a particular radio button (in this case, the one selected) is active, and this explains why you see the same message appear twice.

If all you want to do is perform an action when a particular radio box is selected, then you should consider setting up the callback function to accept a Gtk::RadioButton widget. Using the widget and then checking if it is active should be sufficient for your needs.

Read more here on how to pass information (eg. RadioButton widget) to your callback routine.