实现效果：

点击登录，检验用户密码是否正确，正确则弹出消息框，点击ok转到另一个页面

不正确跳出错误消息框，默认选线为Cancel，点击Yes继续登录

点击Cancel跳出问题消息框，默认选项No，单击yes,退出程序，单击No继续登录

以下是实现的部分代码

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>  //基类QWidget的头文件
#include<QDebug>   //信息调试类，用于输出数据使用
#include<QIcon>
#include<QPushButton>
#include<QLabel>
#include<QLineEdit>
#include<QPixmap>
#include"enter_widget.h"
#include"communication.h"
#include<QMessageBox>
class Widget : public QWidget
{
    Q_OBJECT   //QT的信号与槽元对象

public:
    Widget(QWidget *parent = nullptr);//默认参数构造函数声明
    ~Widget();//虚析构声明
private:
    QLabel *pic;

    QLabel *tx;

    QPushButton* enter;
    QPushButton* login;
    QPushButton* question;

    QLabel* user;
    QLabel* passwd;

    QLineEdit* user_l;
    QLineEdit* passwd_l;

    enter_widget* en;

    QTimer * time;
};
#endif // WIDGET_H

widget.cpp

#include "widget.h"
#include<QDebug>
Widget::Widget(QWidget *parent)
    : QWidget(parent)
{

    qDebug()<<this->size();   //获取当前姐买你的尺寸
    qDebug()<<this->width();  //获取当前组件的宽度
    qDebug()<<this->height(); //获取当前组件的高度
    this->setWindowTitle("QQ");
    //this->resize(500,400);//重新设置界面大小
    //this->resize(QSize(1000,1000));
    this->setFixedSize(500,400);//设置固定的界面大小
    //this->setMaximumSize(600,700);//最大界面大小
    //this->setMinimumSize(200,300);//最小界面大小
    this->setWindowIcon(QIcon("://msg/qq.png"));




    pic = new QLabel(this);
    pic->resize(this->width(),130);
    pic->setPixmap(QPixmap("://msg/fm.png"));
    pic->setScaledContents(true);


    tx = new QLabel(this);

    tx->resize(100,100);
    tx->move(200,60);
    tx->setPixmap(QPixmap("://msg/2.jpg"));
    tx->setScaledContents(true);


    enter = new QPushButton;
    enter->setParent(this);
    enter->setText("登录");
    enter->resize(100,50);
    enter->move(120,300);
    enter->setIcon(QIcon("://msg/enter.png"));
    enter->setStyleSheet("border:0px solid black;border-radius:10;background-color:#FFFFFF");

    login = new QPushButton;
    login->setParent(this);
    login->setText("注册");
    login->resize(100,50);
    login->move(280,300);
    login->setIcon(QIcon("://msg/enter.png"));
    login->setStyleSheet("border:0px solid black;border-radius:10;background-color:#FFFFFF");

//    question = new QPushButton;
//    question->setParent(this);
//    question->setText("问题");
//    question->resize(50,50);
//    question->move(390,300);
//    question->setStyleSheet("border:0px solid black;border-radius:10;background-color:#FFFFFF");

    user = new QLabel;
    user->setParent(this);
    user->setText("用户名:");
    user->resize(60,30);
    user->setAlignment(Qt::AlignCenter);
    //user->setFixedSize(50,20);
    user->move(enter->x(),enter->y()-120);

    passwd = new QLabel(this);
    passwd->setText("密 码:");
    passwd->resize(60,30);
    passwd->setAlignment(Qt::AlignCenter);
    //user->setFixedSize(50,20);
    passwd->move(enter->x(),enter->y()-60);


    user_l = new QLineEdit(this);
    user_l->resize(200,30);
    user_l->move(user->x()+60,user->y());
    user_l->setPlaceholderText("请输入用户名");
    user_l->setStyleSheet("border:0px solid black;border-radius:10;background-color:#b0bfbf");


    passwd_l= new QLineEdit(this);
    passwd_l->resize(200,30);
    passwd_l->move(passwd->x()+60,passwd->y());
    passwd_l->setPlaceholderText("请输入密码");
    passwd_l->setEchoMode(QLineEdit::Password);
    passwd_l->setStyleSheet("border:0px solid black;border-radius:10;background-color:#b0bfbf");

    //切换界面
    //会话界面
    en = new enter_widget;

    //交流界面
    com = new communication;


    connect(enter,&QPushButton::clicked,[=](){

        if(user_l->text()==QString("admin") && passwd_l->text() == "123456")
        {
            QMessageBox succbox(QMessageBox::NoIcon,"登录","登录成功",QMessageBox::Ok,enter);
            int err = succbox.exec();
            if(err == QMessageBox::Ok)
            {
                this->close();
                com->show();
            }
        }
        else
        {
            int err = QMessageBox::critical(
                        this,tr("错误"),
                        tr("账号密码不匹配，是否重新登录"),
                        QMessageBox::Yes | QMessageBox::Cancel,
                        QMessageBox::Cancel
                        );

            if(err == QMessageBox::Cancel)
            {
                int ques = QMessageBox::question(
                            this,
                            tr("问题"),
                            tr("是否退出"),
                            QMessageBox::Yes | QMessageBox::No,
                            QMessageBox::No
                            );
                if(ques == QMessageBox::Yes)
                {
                    this->close();
                }
            }
            this->user_l->clear();
            this->passwd_l->clear();
        }
    });


}
Widget::~Widget()
{
}