Question

I am writing a GTKmm window program; the main window creates two buttons, one for English and one for Chinese. The user can click on the button to bring up a different window in the appropriate language. Currently I am having trouble initializing the multiple-item container inside the main window. It is an object of type MainWindowPane, which inherits Gtk::HBox.

When I try to make, the compiler issues the following error:

$ make 
g++ -g `pkg-config gtkmm-2.4 --cflags` -c MainWindow.cpp  
g++  -g -o QPI_frontend main.o MainWindow.o StartButton.o `pkg-config gtkmm-2.4 --libs`  
MainWindow.o: In function `MainWindow':  
/home/dmurvihill/Documents/QPI_frontend/MainWindow.cpp:9: undefined reference to   `MainWindowPane::MainWindowPane()'  
/home/dmurvihill/Documents/QPI_frontend/MainWindow.cpp:9: undefined reference to  `MainWindowPane::MainWindowPane()'  
collect2: ld returned 1 exit status  
make: *** [QPI_frontend] Error 1  

I am using the latest version of gcc with pkg-config to include the proper libraries. I am also a java person.

/*
 * MAIN_WINDOW.H
 * Responsible for creating "slave" RSED windows. Can create English or Chinese
 * versions of the demo, and can destroy them all with one click.
 */  
#ifndef MAINWINDOW_H  
#define MAINWINDOW_H  

#include <gtkmm/window.h>

//#include "SlaveWindow.h"
#include "StartButton.h"
#include "MainWindowPane.h"

class MainWindow : public Gtk::Window
{
public:
    MainWindow();   
private:
    MainWindowPane pane;
//  std::list<SlaveWindowThread> windows;       // Keeps track of all windows that have been created thus far.
    void destroyAllWindows();           // Iterates through the linked list and destroys each window.
};
#endif      //ifndef MAINWINDOW_H

---

/* 
 * MAIN_WINDOW.CPP
 *
 */  
#include "MainWindow.h"  
#include "MainWindowPane.h"  
#include "StartButton.h"  

MainWindow::MainWindow()// : /*list,*/ pane(/*list*/)
{
    pane;
}

void MainWindow::destroyAllWindows()
{
    //gtk_widget_destroy(*this);
    // TODO: Destroy all the other windows too.
}

---

/*
 * MAIN_WINDOW_PANE.H
 */  
#ifndef MAINWINDOWPANE_H  
#define MAINWINDOWPANE_H  

#include <gtkmm/box.h>
#include <gtkmm/button.h>

//#include "SlaveWindow.h"
#include "StartButton.h"

class MainWindowPane : public Gtk::HBox
{
public:
    MainWindowPane(/*&(std::list)*/);   
private:
    StartButton englishButton;      // Used to create a new RSED demo screen.
    StartButton chineseButton;      // Used to create a new RSED demo in chinese.
//  std::list<SlaveWindow> windows;     // Keeps track of all windows that have been created thus far.
    void destroyAllWindows();       // Iterates through the linked list and destroys each window.
};
#endif      //ifndef MAINWINDOWPANE_H

---

/* 
 * MAIN_WINDOW.CPP
 *
 */  
#include "MainWindowPane.h"  
#include "StartButton.h"  

MainWindowPane::MainWindowPane(/*&(std::list)*/) : 
    englishButton(StartButton::ENGLISH/*,&(std::list)*/), 
    chineseButton(StartButton::CHINESE/*,&(std::list)*/)
{
    pack_start(englishButton);
    englishButton.show();
    pack_start(chineseButton);
    chineseButton.show();
}

void MainWindow::destroyAllWindows()
{
    //gtk_widget_destroy(*this);
    // TODO: Destroy all the other windows too.
}

Answer 1

Undefined reference errors mean you either
forgot to write define the missing function
(by writing an implementation in the .cpp file),
or you forgot to link the appropriate object file or library into the final binary.

In this case, it’s the later reason.
You need to include MainWindowPane.o in the linker command in your makefile:

g++  -g -o QPI_frontend main.o MainWindow.o StartButton.o `pkg-config gtkmm-2.4 --libs`
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                        you need MainWindowPane.o in here