Need help passing a variable to another Form (Signals/Slots)

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Hello,

I've been working on a tool to connect with a MySQL database. However, I'd like to pass an int and an QSqlDatabase object from my mainwindow to another form when a Push Button is clicked.

mainwindow.h

//...
signals:
    void set_mode(int mode, QSqlDatabase db);
//...

mainwindow.cpp

//...
connect(ui->pushButton_open, SIGNAL(&QPushButton::clicked), this, SLOT(&get_info(1, db)));
//...
void MainWindow::on_pushButton_open_clicked()
{
    dialog.setModal(true);
    dialog.exec();
}
//...

dialog.h

//...
public slots:
    void get_info(int x, QSqlDatabase db);
//...

I don't know if I did anything the right way so far. Any idea?

J.Hilk

hi @SnuggleKat
at the very least you're mixing new and old connect syntax,

did you read through this:

http://doc.qt.io/qt-5/signalsandslots.html

and this

https://wiki.qt.io/New_Signal_Slot_Syntax

should give you everything you need to fix your errors.

additionally to that, you're trying to connect to a slot that has incompatible arguments to your signal, that won't work either.

anil_arise

@SnuggleKat try this:
mainwindow.h

//...
//signals:
//void set_mode(int mode, QSqlDatabase db);
//...

mainwindow.cpp

//...
//connect(ui->pushButton_open, SIGNAL(&QPushButton::clicked), this, SLOT(&get_info(1, db)));
//...
void MainWindow::on_pushButton_open_clicked()
{
dialog.setModal(true);
dialog.exec();
dialog.get_info(1, db);
}
//...

dialog.h

//...
public slots:
void get_info(int x, QSqlDatabase db);
//...

OR ***************************************
just change it
connect(ui->pushButton_open, SIGNAL(clicked), dialog, SLOT(get_info(1, db)));

and read it SIGNAL_SLOT

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Thank you for the answers. Gonna give it a try soon. When my MainWindow class inherites from the QMainWindow class, does it indirectly derive from the QObject class?

jsulm

@SnuggleKat said in Need help passing a variable to another Form (Signals/Slots):

When my MainWindow class inherites from the QMainWindow class, does it indirectly derive from the QObject class?

Yes.
You can see what a class is inherited from in the documentation: http://doc.qt.io/qt-5/qwidget.html (as QMainWindow inherits QWidget)
See "Inherits:" column at the top.

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Okay,
inspired by this: http://doc.qt.io/qt-5/signalsandslots.html#a-small-example

I have done the following:

The functions.h defines a class of a QSqlDatabase and an int attribute.

//...
#include <QSqlDatabase>

class Option: QSqlDatabase
{
private:
    QSqlDatabase db;
    int mode;
    
public:
    QSqlDatabase get_db(){ return this->db; }
    int get_mode(){ return this->mode; }
    
signals:
    void send_option(QSqlDatabase db, int mode;{ emit set_up(db, mode); }
    
public slots:
    void set_up(QSqlDatabase db, int mode){ this->db = db; this->mode = mode; }
};

Attempting to pass such an object over to another form by doing this:
mainwindow.h:

//...
#include "functions.h"
//...
private:
    QSqlDatabase db;
    Option opt_main;

mainwindow.cpp

connect(&opt_main, SIGNAL(&Option::send_option), &opt_dia, SLOT(&Option::set_up));

opt_dia is an object of Option. What's the best way to instantiate this option inside the dialog.h? The dialog.h has the functions.h included.

jsulm

@SnuggleKat said in Need help passing a variable to another Form (Signals/Slots):

connect(&opt_main, SIGNAL(&Option::send_option), &opt_dia, SLOT(&Option::set_up));

This is still wrong - you're mixing old and new connect syntax.
Please read the documentation (links were already provided)..

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@jsulm said in Need help passing a variable to another Form (Signals/Slots):

Ah, sorry, totally misseen the new syntacks lacks the SIGNAL and SLOT terms.

QObject::connect(&opt_main, &Option::send_option, &opt_dia, &Option::set_up);

Still something wrong?
What I need to know is what'd be the best way to instantiate the opt_dia object since it's needed in another Form class

mrjj

Hi
you also need
class Option: QSqlDatabase
{
Q_OBJECT
...

Q_OBJECT is needed for signals & slots.

note, after adding it, select clean all, rebuild all to make sure its seen.

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@mrjj said in Need help passing a variable to another Form (Signals/Slots):

Hi
you also need
class Option: QSqlDatabase
{
Q_OBJECT
...

Q_OBJECT is needed for signals & slots.

note, after adding it, select clean all, rebuild all to make sure its seen.

Hello,
just did that.
After this I instantiated another object of the type Option in the dialog.h:

#include <QDialog>
#include "functions.h"

namespace Ui {
class Dialog;
}

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = nullptr);
    Option opt_dia; //note #1
    ~Dialog();

private:
    Ui::Dialog *ui;
    void load_meta();
    int mode;

private slots:
    void on_pushButton_ok_clicked();
};

#1 - This causes an error telling me I have an semantic error on QObject::connect(). Making it static seems to solve this, but then I get the following compiling errors:

no matching function for call to 'QObject::connectImpl(const Object*&, void**, const Object*&, void**, QtPrivate::QSlotObject<void (Option::*)(QSqlDatabase, int), QtPrivate::List<QSqlDatabase, int>, void>*, Qt::ConnectionType&, const int*&, const QMetaObject*)'

invalid static_cast from type 'QObject*' to type 'QtPrivate::FunctionPointer<void (Option::*)(QSqlDatabase, int)>::Object* {aka Option*}'

fyi, the Object class now looks like this:

#include <QtSql>
#include <QSqlDatabase>
#include <QTableView>

class Option: QSqlDatabase
{
    Q_OBJECT
private:
    QSqlDatabase db;
    int mode;

public:
    QSqlDatabase get_db(){return this->db;}
    Option()
    {
        this->db = QSqlDatabase::addDatabase("QMYSQL");
        this->mode = 0;
    }
    virtual ~Option();
    int get_mode(){return this->mode;}

signals:
    void send_option(QSqlDatabase db, int mode){ emit set_up(db, mode); }

public slots:
    void set_up(QSqlDatabase db, int mode){ this->db = db; this->mode = mode; }
};

jsulm

@SnuggleKat said in Need help passing a variable to another Form (Signals/Slots):

This causes an error telling me I have an semantic error on QObject::connect(). Making it static seems to solve this, but then I get the following compiling errors:

Please show your exact connect call

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@jsulm I have found a little mistake. I accidentally assigned the emit call to the signal instead of the slot. After correcting this I tried connecting 2 instances of my class Option inside the MainWindow class which worked (I think). I still can't see an instance of Option outside the MainWindow class.

functions.h

#include <QtSql>
#include <QSqlDatabase>
#include <QTableView>

class Option: public QObject
{
    Q_OBJECT
private:
    QSqlDatabase db;
    int mode;

public:
    QSqlDatabase get_db(){return this->db;}
    int get_mode(){return this->mode;}

signals:
    void send_option(QSqlDatabase db, int mode);

public slots:
    void set_up(QSqlDatabase db, int mode){ this->db = db; this->mode = mode; emit send_option(db, mode);}
};

mainwindow.h

//...
class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private slots:
    void on_pushButton_connect_clicked();
    void on_pushButton_create_pdf_clicked();
    void on_pushButton_save_settings_clicked();
    void on_pushButton_open_clicked();

private:
    Ui::MainWindow *ui;
    int create_pdf();
    Dialog dialog;
    QSqlDatabase db;
    Option opt_main, testopt; //testopt was the test-instance i used
};

mainwindow.cpp

//...
QObject::connect(&opt_main, &Option::send_option, &opt_dia, &Option::set_up); //opt_dia not declared in this scope
//...
void MainWindow::on_pushButton_open_clicked()
{
    opt_main.send_option(db, 123);
    dialog.setModal(true);
    dialog.exec();
}

dialog.h

//...
#include "functions.h"

namespace Ui {
class Dialog;
}

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = nullptr);
    //QObject* x;
    Option opt_dia;
    ~Dialog();

private:
    Ui::Dialog *ui;
    void load_meta();
    int mode;

private slots:
    void on_pushButton_ok_clicked();
};

dialog.cpp

//...
void Dialog::on_pushButton_ok_clicked()
{
    qDebug() << "mode: " + QString::number(opt_dia.get_mode()) + " - " + opt_dia.get_db().userName(); //testing the result
}

opt_dia cannot be found since it's not declared in this scope. If I use &Dialog::opt_dia it is found but the connection will fail to compile with the following error: Fehler: no matching member function for call to 'connect'

jsulm

@SnuggleKat Why don't you simply define the slot inside Dialog class?
Exposing a member (opt_dia) as public to outside world is bad design.

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@jsulm said in Need help passing a variable to another Form (Signals/Slots):

@SnuggleKat Why don't you simply define the slot inside Dialog class?
Exposing a member (opt_dia) as public to outside world is bad design.

I'm aware it's bad design. That's why I asked what'd be the best way to instantiate it.
Is it the right way to access the signal through opt_dia if I define the slot inside the Dialog class?
And what should the connect() look like now?

jsulm

@SnuggleKat said in Need help passing a variable to another Form (Signals/Slots):

When I define the slot inside the Dialog class is it the right way to access the signal through opt_dia?

I don't understand this - what signal do you want to access from opt_dia?
Connect would be:

connect(&opt_main, &Option::send_option, &dialog, &Dialog::set_up);

Actually I don't see a need for signal/slot here as dialog is a member of MainWindow. set_up can be a normal method which you call like any other without emiting a signal:

dialog.set_up(...);

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@jsulm Sorry, I meant to ask if ithis would be the right way of doing it:

public slots:
   void set_up(QSqlDatabase db, int mode){ this->db = db; this->mode = mode; emit opt_dia.send_option(db, mode);}

Change the connect() method as suggestion but the values don't get passed over.
Same for adding dialog.set_up();
Should I make QSqlDatabase db and int mode static?

jsulm

@SnuggleKat "Change the connect() method as suggestion but the values don't get passed over." - please show the code.
"Should I make QSqlDatabase db and int mode static?" - please don't! Static variables are bad design in most cases.

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@jsulm said in Need help passing a variable to another Form (Signals/Slots):

@SnuggleKat "Change the connect() method as suggestion but the values don't get passed over." - please show the code.
"Should I make QSqlDatabase db and int mode static?" - please don't! Static variables are bad design in most cases.

That's what I thought.

void MainWindow::on_pushButton_open_clicked()
{
    opt_main.send_option(db, 123);
    QObject::connect(&opt_main, &Option::send_option, &dialog, &Dialog::set_up);
    dialog.set_up(db, 123); //also tried this which didn't help either
    dialog.setModal(true);
    dialog.exec();
}

jsulm

@SnuggleKat I really don't get why you use signals slots here?

void MainWindow::on_pushButton_open_clicked()
{
    dialog.set_up(db, 123); //also tried this which didn't help either
    dialog.setModal(true);
    dialog.exec();
}

"//also tried this which didn't help either" - please show what dialog.set_up(db, 123) is doing.

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@jsulm
qDebug() shows me that it effectively does nothing. dialog.set_up() does not touch the object declared as opt_dia.

"mode: 12141696 - "

12141696 seems to be a random value.
This is the qDebug() line:

qDebug() << "mode: " + QString::number(opt_dia.get_mode()) + " - " + opt_dia.get_db().userName();