How to access a variable from another class using Qt C++? [duplicate]

Question

I've been trying to access another class variable, but I can't build the project. QtCreator doesn't show any error / alert. I'm trying to create a ToDo app as a University project and I need to access the name of the list to link the task to the list.

I'm sorry for this very simple question but I'm starting out both with C++ and Qt. I have already checked similar questions, but none of the answers work for me (100% my fault).

These are some code snippets:

ListManager.h

public:
    static QStringList listName;

ListManager.cpp

void ListManager::on_pushButton_addList_clicked(){
    QString nameList = QInputDialog::getText(this, tr("Add list"), tr("List name"), QLineEdit::Normal, tr("Untitled list"), &ok);
    listName += nameList; // saves the name of the list in a QStringList
}

In the class TaskManager.cpp I want to access the names to set the QComboBox labels.

TaskManager.cpp

#include "listmanager.h"
// more code
void ListManager::on_pushButton_addTask_clicked(){
    QStringList listsNames = ListManager::listName;
    addTaskDialog.comboBox->addItems(listsNames);
}

As I mentioned before I don't get any error / alert by both QtCreator & CppCheck. When I try to build the program I get these issues:

error: Undefined symbols for architecture arm64:
  "ListManager::listName", referenced from:
      ListManager::on_pushButton_addList_clicked() in listmanager.o
      TaskManager::on_pushButton_addTask_clicked() in taskmanager.o
error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [ToDoApp.app/Contents/MacOS/ToDoApp] Error 1

Any help would be much appreciated!

Answer 1

To your ListManager.cpp add a line (usually it is placed before any method implementaton) like the following

QStringList ListManager::listName;

This will instantiate the static variable. The declaration in the header file is just a declaration, then you have to create it somewhere in the code.

If you don't istantiate the variable the compiler will not give you any error but the linker does because it is not able to find the real istance of the variable.

I'd like to add a suggestion: avoid where possible the use of static variables. It's better that the external object TaskManager holds a reference to ListManager and use a data member,