Signal/slot between widgets

[d_stranz]

emit(transfer(testo));
connect(this, SIGNAL(trasfer(QString)), form, SLOT(UpdateField(QString)));

This is backwards. If you emit the signal before you connect the other window's slot to it, how will the other window receive it? The signal doesn't hang around, asking "Hey, anybody listening? Hello? Hello? Anybody?" until a slot eventually gets connected. Instead, it says, "Hey, anybody listening? No? Then I'm out of here."

Your code has a serious memory leak. Each time the button is clicked, you create a new Page2 instance, completely unrelated to and independent of any others created during previous clicks. Even if you eventually hide them, they are still around, lurking invisibly in the background.

A better solution is to create a Page2 instance in the Page1 constructor (as a child of the Page1 instance: page2 = new Page2( this )) and store the pointer as a member variable of Page1 Page2 * page2). When you create it you also make the connection between your signal and the Page2 slot. Once. In the Page1 constructor.

In your button click slot, you can simply call page2->show() and then emit your signal. Or vice-versa. The Page2 widget instance will receive the signal whether it is visible or not. Because you create the Page2 widget with the Page1 instance as its parent, it will be deleted automatically when the Page1 widget is deleted.

[sr1s]

Thank you very much for your explanation! Now it's clear the way to work. By now, I have this problem: when the child page2=new Page2 (this) is created, it superimposes itself to the Page1, and, as soon as I push the button, I obtain a "mixed" window with Page1 and Page2 widgets together. I can't hide the Page1, because so I remove the whole window... perhaps I have to use different solution than Qwidget-with-Qwidget? Thanks a lot!

[d_stranz]

I obtain a "mixed" window with Page1 and Page2 widgets together.

Ah, yes, that's probably right. If you are not using a layout on Page1, then it will merge Page2 into Page1 when it is shown. OK, when you create page2, do not make it a child of page1 (this). In that case, you need to make sure you delete it in the Page1 destructor, just like you delete the ui pointer.

[sr1s]

SOLVED...just page2=new Page2 ();
If someother newbe as me is interested, I resend the code:

Pag1.h

#ifndef PAGE1_H
#define PAGE1_H
 
#include <QWidget>
#include  "page2.h"
 
namespace Ui {
class Page1;
}
 
class Page1 : public QWidget
{
    Q_OBJECT
 
public:
    explicit Page1(QWidget *parent = 0);
    ~Page1();
 
signals:
    void transfer(QString);
 
private:
    Ui::Page1 *ui;
    Page2* page2;
 
private slots:
    void on_pushButton_clicked();
 
};
 
#endif // PAGE1_H

#ifndef PAGE1_H
#define PAGE1_H

#include <QWidget>
#include  "page2.h"

namespace Ui {
class Page1;
}

class Page1 : public QWidget
{
    Q_OBJECT

public:
    explicit Page1(QWidget *parent = 0);
    ~Page1();

signals:
    void transfer(QString);

private:
    Ui::Page1 *ui;
    Page2* page2;

private slots:
    void on_pushButton_clicked();

};

#endif // PAGE1_H

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Page1.cpp

#include "page1.h"
#include "ui_page1.h"
#include "page2.h"
#include <qdebug.h>
 
Page1::Page1(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Page1)
{
    ui->setupUi(this);
    page2=new Page2;
    connect(this, SIGNAL(transfer(QString)), page2, SLOT(UpdateField(QString)));
}
 
Page1::~Page1()
{
    delete ui;
}
 
void Page1::on_pushButton_clicked()
{
    QString testo;
    testo=ui->to_pass->text();
    emit(transfer(testo));
    page2->show();
    }

#include "page1.h"
#include "ui_page1.h"
#include "page2.h"
#include <qdebug.h>

Page1::Page1(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Page1)
{
    ui->setupUi(this);
    page2=new Page2;
    connect(this, SIGNAL(transfer(QString)), page2, SLOT(UpdateField(QString)));
}

Page1::~Page1()
{
    delete ui;
}

void Page1::on_pushButton_clicked()
{
    QString testo;
    testo=ui->to_pass->text();
    emit(transfer(testo));
    page2->show();
    }

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Page2.h and Page2.cpp were not changed.
Thanks again!

[d_stranz]

Still have a small memory leak - you create page2 but never delete it. Change the Page1 destructor to this:

Page1::~Page1()
{
    delete ui;
    delete page2;    // <-- add
}

Page1::~Page1()
{
    delete ui;
    delete page2;    // <-- add
}

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[sr1s]

You are right!
Thank you for your help, d_stranz, and have a nice weekend!

[ado130]

d_stranz: What about this solution?

void Page1::on_pushButton_clicked()
{
    Page2 *form = new Page2();
    form->setAttribute(Qt::WA_DeleteOnClose); // added this line
    connect(this, SIGNAL(transfer(QString)), form, SLOT(UpdateField(QString)));
    QString testo=ui->to_pass->text();
    emit(transfer(testo));    
    form->show();
}

void Page1::on_pushButton_clicked()
{
    Page2 *form = new Page2();
    form->setAttribute(Qt::WA_DeleteOnClose); // added this line
    connect(this, SIGNAL(transfer(QString)), form, SLOT(UpdateField(QString)));
    QString testo=ui->to_pass->text();
    emit(transfer(testo));    
    form->show();
}

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I understand your solution, in general I do not have problem with that, but I think this looks "better" - at least you do not have to remember to delete it in destructor.

[d_stranz]

What about this solution?

If you want a completely new Page2 instance every time you click the button, this is one way to do it that avoids a member variable. It depends on how complex Page2 really is and whether constructing it on-the-fly like this would cause a noticeable pause between the click and the show.

On the other hand, if you want the Page2 instance to "remember" what it was displaying the last time it was used, this approach will not be very good. Making a new instance each time will always initialize it to a fresh state with no memory of what was there before. So, if Page2 is used for setting options, with check boxes, radio buttons, and so forth, having them reset to the default settings each time will not make users very happy. If you have a single instance and simply show() and hide() it, then it will keep its state between uses.

My preference is to do as I suggested: create a single instance in the constructor and re-use it when needed. If you do need to set it to a default state, then add a method to the Page2 class to do that and call it before you show it.

[sr1s]

It's a really good solution!
My first problem was simply to make it working, but of course the second step is "make it work better"...
With your second suggestion I can optimize my code, and now I'm trying to create a more complex Page2 for better understanding the differences of two ways.
Have a nice day