练习：

#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    //去掉头部
    this->setWindowFlag(Qt::FramelessWindowHint);
    //去掉空白部分
    this->setAttribute(Qt::WA_TranslucentBackground);
}

Widget::~Widget()
{
    delete ui;
}


void Widget::on_pushButton_2_clicked()
{
    this->close();
}

void Widget::on_pushButton_3_clicked()
{
    this->showMinimized();
}

void Widget::on_pushButton_clicked()
{
    if(ui->lineEdit->text()=="admin" && ui->lineEdit_2->text()=="123456"){
        this->close();
        emit this->signal();
    }
    else{
        QMessageBox::information(this,"Error","密码或账户名错误");
        ui->lineEdit->clear();
        ui->lineEdit_2->clear();
    }
}

#include "seon.h"
#include "ui_seon.h"

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

Seon::~Seon()
{
    delete ui;
}

void Seon::my_slots()
{
    this->show();
}

#include "widget.h"
#include "Seon.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.show();
    Seon s;
    QObject::connect(&w,&Widget::signal,&s,&Seon::my_slots);
    return a.exec();
}

初始界面： 

账号或密码错误提示：

成功跳转界面：

思维导图：