main.cpp

#include "myapp.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    myapp w;
    w.show();
    return a.exec();
}

myapp.h

#ifndef MYAPP_H
#define MYAPP_H

#include <QWidget>
#include <QHBoxLayout>
#include <QPushButton>
#include "login.h"

class myapp : public QWidget
{
    Q_OBJECT

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

public:
    QHBoxLayout *mybox;
    QPushButton *mybtn;
    login *l;
public slots:
    void go_log();
signals:
    void go(QString);
};
#endif // MYAPP_H
 

myapp.cpp

#include "myapp.h"

myapp::myapp(QWidget *parent)
    : QWidget(parent)
{
    this->mybox = new QHBoxLayout;
    this->mybtn = new QPushButton;

    this->mybox->addWidget(mybtn);
    this->setLayout(mybox);
    connect(this->mybtn,&QPushButton::clicked,this,&myapp::go_log);
}

myapp::~myapp()
{
}

void myapp::go_log()
{
    this->l = new login;
    l->show();
    connect(this,&myapp::go,this->l,&login::go);
    emit go("123");
    this->close();
}
 

login.h

#ifndef LOGIN_H
#define LOGIN_H

#include <QWidget>
#include <QHBoxLayout>
#include <QLabel>

class login : public QWidget
{
    Q_OBJECT
public:
    explicit login(QWidget *parent = nullptr);
public:
    QHBoxLayout *lbox_x;
    QLabel *lab;
public slots:
    void go(QString);

};

#endif // LOGIN_H
 

login.cpp

#include "login.h"

login::login(QWidget *parent) : QWidget(parent)
{

}

void login::go(QString str)
{
    this->lbox_x = new QHBoxLayout;
    this->lab = new QLabel;

    this->lab->setText(str);
    this->lbox_x->addWidget(lab);

    this->setLayout(lbox_x);
}