gtkmm链接器错误

你好，stackoverflow：），我正在编译一个基本的gui程序，它取2个双的和并打印出来。我的代码实际上很简单，我将首先发布我通过命令行编译的3个文件

"g++ main.cc examplewindow.cc -o main `pkg-config gtkmm-3.0 --cflags --libs`"

命令行上的错误为：

/tmp/ccSeXudb.o: In function `ExampleWindow::on_button_clicked_calculate()': examplewindow.cc:(.text+0x1e70): undefined reference to `ExampleWindow::double_to_ustring(double)'
collect2: error: ld returned 1 exit status

这是3个文件：

                                    examplewindow.h
#ifndef GTKMM_EXAMPLEWINDOW_H
#define GTKMM_EXAMPLEWINDOW_H
#include <gtkmm.h>
#include <boost/lexical_cast.hpp>
#include <stdlib.h>
#include <string>
class ExampleWindow : public Gtk::Window
{
public:
  ExampleWindow();
  virtual ~ExampleWindow();
protected:
  //Signal handlers:
  void on_checkbox_editable_toggled();
  void on_checkbox_visibility_toggled();
  Glib::ustring get_entry1();
  Glib::ustring get_entry2();
  std::string double_to_ustring(double a);
  void on_button_clicked_calculate();
  void on_button_close();
  //Child widgets:
  //Gtk::Box m_HBox;
  Gtk::Box m_VBox;
  Gtk::Box m_ZBox;
  Gtk::Entry m_Entry, m_Entry2, m_Entry3; 
  Gtk::Button m_Button_Close, m_Button_Calculate;
  //Gtk::CheckButton m_CheckButton_Editable, m_CheckButton_Visible;
  Gtk::Label m_Label_Sum, m_Label_Equals;
};
#endif //GTKMM_EXAMPLEWINDOW_H


                                     main.cc
#include "examplewindow.h"
#include <gtkmm/application.h>
int main(int argc, char *argv[])
{
  Glib::RefPtr<Gtk::Application> app = Gtk::Application::create(argc, argv,     "org.gtkmm.example");
  ExampleWindow window;
  //Shows the window and returns when it is closed.
  return app->run(window);
}


                                  examplewindow.cc
#include "examplewindow.h"
#include <iostream>
#include <boost/lexical_cast.hpp>
#include <stdlib.h>
#include <string>

ExampleWindow::ExampleWindow()
: m_VBox(Gtk::ORIENTATION_VERTICAL),
  m_ZBox(Gtk::ORIENTATION_HORIZONTAL),
  m_Button_Calculate("Calculate"),
  m_Button_Close("Close")
{
  set_size_request(200, 100);
  set_title("Trial Version:P");
  add(m_VBox);

  m_VBox.add(m_ZBox);
  m_Entry.set_max_length(50);
  m_Entry.set_text("First");
  m_Entry.set_text(m_Entry.get_text() + " value");
  m_Entry.select_region(0, m_Entry.get_text_length());
  m_ZBox.pack_start(m_Entry);
  m_Label_Sum.set_text("+");
  m_ZBox.pack_start(m_Label_Sum); 
  m_Entry2.set_max_length(50);
  m_Entry2.set_text("Second");
  m_Entry2.set_text(m_Entry2.get_text() + " value");
  m_Entry2.select_region(0, m_Entry2.get_text_length());
  m_ZBox.pack_start(m_Entry2);
  m_Label_Equals.set_text("=");
  m_ZBox.pack_start(m_Label_Equals);
  m_Entry3.set_max_length(50);
  m_Entry3.set_text("The");
  m_Entry3.set_text(m_Entry3.get_text() + " result"); 
  m_Entry3.set_editable(0); 
  m_Entry3.select_region(0, m_Entry3.get_text_length());
  m_ZBox.pack_start(m_Entry3);

  m_Button_Calculate.signal_clicked().connect( sigc::mem_fun(*this,     &ExampleWindow::on_button_clicked_calculate) );
  m_VBox.pack_start(m_Button_Calculate);
  m_Button_Calculate.set_can_default();
  m_Button_Calculate.grab_default();
  m_Button_Close.signal_clicked().connect( sigc::mem_fun(*this,
              &ExampleWindow::on_button_close) );
  m_VBox.pack_start(m_Button_Close);
  m_Button_Close.set_can_default();
  m_Button_Close.grab_default();
  show_all_children();
}
ExampleWindow::~ExampleWindow()
{
}
Glib::ustring ExampleWindow::get_entry1()
{
  return m_Entry.get_text();
}
Glib::ustring ExampleWindow::get_entry2()
{
  return m_Entry2.get_text();
}
std::string double_to_ustring(double a)
{
  std::string str = boost::lexical_cast<std::string>(a);
  return str; 
}
void ExampleWindow::on_button_clicked_calculate()
{ 
  Glib::ustring a = get_entry1();
  Glib::ustring b = get_entry2();
  double m = std::atof( a.c_str() ) + std::atof( b.c_str() );
  Glib::ustring z = double_to_ustring(m);
  m_Entry3.set_text(z);
}
void ExampleWindow::on_button_close()
{
  hide();
}

我已经做了一个只有c++的版本：

#include <iostream>
#include <boost/lexical_cast.hpp>
#include <stdlib.h>
#include <string>
using namespace std;
string double_to_string(double a)
{
  string str = boost::lexical_cast<std::string>(a);
  return str; 
}
int main()
{
  string a="3", b="5";
  double m = atof( a.c_str() ) + atof( b.c_str() );
  string z = double_to_string(m);
  cout << z << endl;
}

c++版本有效，所以我需要帮助。如何使链接器工作？