RadioButton
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## RadioButton

Like checkboxes, radio buttons also inherit from `Gtk::ToggleButton`, but these work in groups, and only one RadioButton in a group can be selected at any one time.

### Groups

There are two ways to set up a group of radio buttons. The first way is to create the buttons, and set up their groups afterwards. Only the first two constructors are used. In the following example, we make a new window class called `RadioButtons`, and then put three radio buttons in it:

      class RadioButtons : public Gtk::Window
      {
      public:
          RadioButtons();

      protected:
          Gtk::RadioButton m_rb1, m_rb2, m_rb3;
      };

      RadioButtons::RadioButtons()
        : m_rb1("button1"),
          m_rb2("button2"),
          m_rb3("button3")
      {
          Gtk::RadioButton::Group group = m_rb1.get_group();
          m_rb2.set_group(group);
          m_rb3.set_group(group);
      }

We told gtkmm to put all three `RadioButton`s in the same group by obtaining the group with `get_group()` and using `set_group()` to tell the othe `RadioButton`s to share that group.

Note that you can't just do

    m_rb2.set_group(m_rb1.get_group()); //doesn't work

because the group is modified by `set_group()` and therefore non-const.

The second way to set up radio buttons is to make a group first, and then add radio buttons to it. Here's an example:

      class RadioButtons : public Gtk::Window
      {
      public:
          RadioButtons();
      };

      RadioButtons::RadioButtons()
      {
          Gtk::RadioButton::Group group;
          Gtk::RadioButton *m_rb1 = manage( new Gtk::RadioButton(group,"button1"));
          Gtk::RadioButton *m_rb2 = manage( new Gtk::RadioButton(group,"button2"));
          Gtk::RadioButton *m_rb3 = manage( new Gtk::RadioButton(group,"button3"));
      }

We made a new group by simply declaring a variable, `group`, of type `Gtk::RadioButton::Group`. Then we made three radio buttons, using a constructor to make each of them part of `group`.

### Methods

`RadioButtons` are "off" when created; this means that when you first make a group of them, they will all be off. Don't forget to turn one of them on using `set_active()`:

Reference

### Example

The following example demonstrates the use of `RadioButton`s:

Figure 4.3. RadioButton

Source Code

File: radiobuttons.h

    #ifndef GTKMM_EXAMPLE_RADIOBUTTONS_H
    #define GTKMM_EXAMPLE_RADIOBUTTONS_H

    #include <gtkmm/window.h>
    #include <gtkmm/radiobutton.h>
    #include <gtkmm/box.h>
    #include <gtkmm/separator.h>

    class RadioButtons : public Gtk::Window
    {
    public:
      RadioButtons();
      virtual ~RadioButtons();

    protected:
      //Signal handlers:
      virtual void on_button_clicked();

      //Child widgets:
      Gtk::VBox m_Box_Top, m_Box1, m_Box2;
      Gtk::RadioButton m_RadioButton1, m_RadioButton2, m_RadioButton3;
      Gtk::HSeparator m_Separator;
      Gtk::Button m_Button_Close;
    };

    #endif //GTKMM_EXAMPLE_RADIOBUTTONS_H

File: main.cc

    #include <gtkmm/main.h>
    #include "radiobuttons.h"

    int main(int argc, char *argv[])
    {
      Gtk::Main kit(argc, argv);

      RadioButtons buttons;
      Gtk::Main::run(buttons); //Shows the window and returns when it is closed.

      return 0;
    }

File: radiobuttons.cc

    #include "radiobuttons.h"

    RadioButtons::RadioButtons() :
      m_Box1(false, 10),
      m_Box2(false, 10),
      m_RadioButton1("button1"),
      m_RadioButton2("button2"),
      m_RadioButton3("button3"),
      m_Button_Close("close")
    {
      set_title("radio buttons");
      set_border_width(0);

      //Put radio buttons 2 and 3 in the same group as 1:
      Gtk::RadioButton::Group group = m_RadioButton1.get_group();
      m_RadioButton2.set_group(group);
      m_RadioButton3.set_group(group);

      add(m_Box_Top);

      //Put Box1 in m_Box_Top:
      m_Box1.set_border_width(10);
      m_Box_Top.pack_start(m_Box1);

      //Put the radio buttons in Box1:
      m_Box1.pack_start(m_RadioButton1);
      m_Box1.pack_start(m_RadioButton2);
      m_Box1.pack_start(m_RadioButton3);
      m_RadioButton2.set_active();

      //Add a separator:
      m_Box_Top.pack_start(m_Separator, Gtk::PACK_SHRINK);

      //Put Box2 in m_Box_Top:
      m_Box2.set_border_width(10);
      m_Box_Top.pack_start(m_Box2, Gtk::PACK_SHRINK);

      //Put Close button in Box2:
      m_Box2.pack_start(m_Button_Close);
      m_Button_Close.set_flags(Gtk::CAN_DEFAULT);
      m_Button_Close.grab_default();

      m_Button_Close.signal_clicked().connect( sigc::mem_fun(*this, &RadioButtons::on_button_clicked) );

      show_all_children();
    }

    RadioButtons::~RadioButtons()
    {
    }

    void RadioButtons::on_button_clicked()
    {
      hide(); //to close the application.
    }

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