jump.h

#ifndef JUMP_H
#define JUMP_H

#include <QWidget>

namespace Ui {
class Jump;
}

class Jump : public QWidget
{
    Q_OBJECT

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

#endif // JUMP_H

widget.h

#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_pushButton_clicked();

    void on_cancel_clicked();

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

jump.cpp 

#include "jump.h"
#include "ui_jump.h"

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

void Jump::jumps()
{
    this->show();
}
Jump::~Jump()
{
    delete ui;
}

main.cpp

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

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    Jump p;
    QObject::connect(&w,&Widget::my_jump,&p,&Jump::jumps);
    w.show();
    return a.exec();
}

 widget.h

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

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    this->setWindowFlag(Qt::FramelessWindowHint);
    this->setAttribute(Qt::WA_TranslucentBackground);

}

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


void Widget::on_pushButton_clicked()
{
    if(ui->username->text() == "admin" && ui->password->text() == "123456")
    {
        QMessageBox msg(QMessageBox::Information,
                        "提示",
                        "登录成功",
                        QMessageBox::Yes | QMessageBox::No,
                        this);
        int rec = msg.exec();
        if(rec == QMessageBox::Yes)
        {
            this->close();
            emit my_jump();
        }
    }
    else
    {
        int ret = QMessageBox::critical(this,"错误","账号或密码不匹配，请重新输入",
                                QMessageBox::Yes | QMessageBox::No);
        if(ret == QMessageBox::Yes)
        {
            ui->username->clear();
            ui->password->clear();
        }
        else if(ret == QMessageBox::No)
        {
            this->close();
        }
    }

}

void Widget::on_cancel_clicked()
{
    int canret = QMessageBox::question(this,"问题","您确定要退出吗?",
                          QMessageBox::Yes | QMessageBox::No);
    if(canret == QMessageBox::Yes)
    {
        this->close();
    }
}