c++ - QT C++中对全局变量的 undefined reference

Question

我尝试将字符串值从mainwindow.cpp传递到userdetails.cpp。我曾经使用过全局变量。并且在程序运行时，它显示错误消息“对globelusername 的 undefined reference ”。 globelusername表示全局变量名称。代码中有什么错误？

主窗口

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H

    #include <QMainWindow>
    #include "register.h"
    #include "userdetails.h"
    //#include <QtSql>
    //#include <QSqlDatabase>
    //#include <QMessageBox>

    extern QString globelusername;

    QT_BEGIN_NAMESPACE
    namespace Ui { class MainWindow; }
    QT_END_NAMESPACE

    class MainWindow : public QMainWindow
    {
        Q_OBJECT

    public:
        MainWindow(QWidget *parent = nullptr);
        ~MainWindow();

主窗口

    #include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    ui ->loginusername->setPlaceholderText("Username");
    ui ->loginpassword->setPlaceholderText("Password");
    QString username = ui ->loginusername ->text();
    QString globelusername = username;
     return ;//
}

MainWindow::~MainWindow()
{
    delete ui;

    }  

void MainWindow::on_pushButton_clicked()
{
        //open new registration in new window
    hide();
    regist = new Register(this);
    regist ->show();

}

void MainWindow::on_pushButton_2_clicked()
{
    //database connection
     .....

     QString username = ui ->loginusername ->text();
     QString password = ui ->loginpassword ->text();

     if(db.open()){

         //query create

         QSqlQuery query(QSqlDatabase::database("MyConnect"));

         query.prepare(QString("SELECT * FROM user_reg_elec WHERE username = :username AND password = :password"));

         query.bindValue(":username", username);
         query.bindValue(":password", password);

         QString globelusername = username; //globlevariable
       }

userdetails.cpp

#include "userdetails.h"
#include "ui_userdetails.h"
#include <QSqlError>
#include "mainwindow.h"

Userdetails::Userdetails(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Userdetails)

{
    ui->setupUi(this);

}

Userdetails::~Userdetails()
{

}

void Userdetails::on_pushButton_4_clicked()
{

    {
            // database connection
            ........

            QString abc = globelusername;

Answer 1

这条线

extern QString globelusername;

只是声明一个全局变量而没有定义它。

您必须在一个.cpp文件中定义它(例如在mainwindow.cpp中):

#include "mainwindow.h"
#include "ui_mainwindow.h"

QString globelusername;

// ...