button clicked to set text in another form

[nix]

No, i'm in the same case. The pushbutton in the mainwindow just shows the second window (form1). The second window has its own pushbutton just like you.
How does you second window is created and shown?

Complete code :
mainwindow.c

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
#include <form1.h>
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(openWindow()));
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
void MainWindow::openWindow()
{
    Form1 *form = new Form1(this);
    connect(form, SIGNAL(transfertCompleted(QString)), this, SLOT(updateLabel(QString)));
    form->show();
}
 
void MainWindow::updateLabel(QString text)
{
   ui->label->clear();
   ui->label->setText(text);
}

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <form1.h>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(openWindow()));
}

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

void MainWindow::openWindow()
{
    Form1 *form = new Form1(this);
    connect(form, SIGNAL(transfertCompleted(QString)), this, SLOT(updateLabel(QString)));
    form->show();
}

void MainWindow::updateLabel(QString text)
{
   ui->label->clear();
   ui->label->setText(text);
}

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form1.c

#include "form1.h"
#include "ui_form1.h"
 
Form1::Form1(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Form1)
{
    ui->setupUi(this);
}
 
Form1::~Form1()
{
    delete ui;
}
 
void Form1::on_pushButton_clicked()
{
//write data
//i want when click it send data and also send text to mainform
    emit(transfertCompleted("transfer complete"));
}

#include "form1.h"
#include "ui_form1.h"

Form1::Form1(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Form1)
{
    ui->setupUi(this);
}

Form1::~Form1()
{
    delete ui;
}

void Form1::on_pushButton_clicked()
{
//write data
//i want when click it send data and also send text to mainform
    emit(transfertCompleted("transfer complete"));
}

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#ifndef FORM1_H
#define FORM1_H
 
#include <QDialog>
 
namespace Ui {
class Form1;
}
 
class Form1 : public QDialog
{
    Q_OBJECT
 
public:
    explicit Form1(QWidget *parent = 0);
    ~Form1();
signals:
    void transfertCompleted(QString);
 
private:    
    Ui::Form1 *ui;
private slots:
    void on_pushButton_clicked();
};
 
#endif // FORM1_H

#ifndef FORM1_H
#define FORM1_H

#include <QDialog>

namespace Ui {
class Form1;
}

class Form1 : public QDialog
{
    Q_OBJECT
    
public:
    explicit Form1(QWidget *parent = 0);
    ~Form1();
signals:
    void transfertCompleted(QString);

private:    
    Ui::Form1 *ui;
private slots:
    void on_pushButton_clicked();
};

#endif // FORM1_H

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#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
 
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
private slots:
    void openWindow();
    void updateLabel(QString text);
 
};
 
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT
    
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    
private:
    Ui::MainWindow *ui;
private slots:
    void openWindow();
    void updateLabel(QString text);

};

#endif // MAINWINDOW_H

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[vanduongbk]

hi ,i compiler and have error as:
mainscreen.cpp:180: error: C2039: 'pushButton' : is not a member of 'Ui::MainScreen'
how to resolve it

[nix]

Im my case I use a mainwindow with a pushbutton. A press on that button creates the form1 and shows it. Another click in pushbutton from the form1 change the content of the label in mainwindow. I supposed that it was the way you did it, but it may be not.
Maybe you do not need this behavior.

I still don't get how you create your form1, the problem is probably around here. How does your mainwindow know about form1 in you code???
Are you creating both windows in your main function? In that case move the connect between both windows to the main fonction too.

[vanduongbk]

hi ,
i creat form1 only with pushButton to send data and i want to do when i click pushbutton in form1 ,it also simultaneously display with content a text that "transfer complete" on Qlabel in mainwindow
plz help me in a demo code

[nix]

This is quick & dirty, just demo, it is how i understand your problem.
testqt.tar.gz

[vanduongbk]

hello nix
your code is run ok
but i want to ask how i do not need show form1 ,only click in form1 then back to mainform and a text on mainform is changed without show form1
thank

[nix]

but i want to ask how i do not need show form1 ,only click in form1 then back to mainform and a text on mainform is changed without show form1

I'm sorry but I don't understand your problem.
You want click on form1, so it have to be shown. If you just want form1 to not be visible after you have pressed the button and change the text into the mainwindow, just connect the clicked() signal of your form1 button to the close() slot of form1.
But this is probably not what you meant, in that case please try to give a complete description of what you are trying to do, especially when windows are created, shown and hide.