头文件：Second

#ifndef SECOND_H
#define SECOND_H

#include <QWidget>

namespace Ui {
class Second;
}

class Second : public QWidget
{
    Q_OBJECT

public:
    explicit Second(QWidget *parent = nullptr);
    ~Second();
public slots:
    void jump_slot();
private:
    Ui::Second *ui;
};

#endif // SECOND_H

头文件：Widget

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QMessageBox>

QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();
signals:
    void my_jump();
private slots:
    void on_loginBtn_clicked();

    void on_cancelBtn_clicked();

private:
    Ui::Widget *ui;
};
#endif // WIDGET_H

main.cpp

#include "widget.h"
#include "second.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.show();

    Second s;

    QObject::connect(&w,&Widget::my_jump,&s,&Second::jump_slot);

    return a.exec();
}

Widget.cpp

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

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

    this->setWindowFlag(Qt::FramelessWindowHint);//设置纯净窗口
    this->setAttribute(Qt::WA_TranslucentBackground);//去掉窗口其他部分
}

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

//登录按钮
void Widget::on_loginBtn_clicked()
{
    if(ui->userNameEdit->text()=="admin" && ui->passwdEdit->text()=="123456")
    {
        int ret = QMessageBox::information(this,"登陆","登陆成功",QMessageBox::Ok);

        if(ret==QMessageBox::Ok)
        {
            emit my_jump();
            this->close();
        }
    }
    else
    {
        QMessageBox msg(QMessageBox::Warning,"错误","账号和密码不匹配",QMessageBox::Yes|QMessageBox::No,this);
        int ret1 = msg.exec();
        if(ret1==QMessageBox::Yes)
        {
            ui->passwdEdit->clear();
        }
        else if(ret1==QMessageBox::No)
        {
            this->close();
        }
    }

}

void Widget::on_cancelBtn_clicked()
{
    int res = QMessageBox::question(this,"问题","您是否确定要退出登录?",QMessageBox::Yes|QMessageBox::No);
    if(res==QMessageBox::Yes)
    {
        this->close();
    }
    else if(res==QMessageBox::No)
    {

    }
}

Second.cpp

#include "second.h"
#include "ui_second.h"

Second::Second(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Second)
{
    ui->setupUi(this);
    this->setWindowFlag(Qt::FramelessWindowHint);//设置纯净窗口
}

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

void Second::jump_slot()
{
    this->show();
}

效果图

思维导图