Problems Qt Widgets applications and MySql

[Emit]

This is just a part of the program. I don't understand why doesn't it throws the error

'mojabaza' was not declared in this scope

'mojabaza' was not declared in this scope

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MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    mojabaza=QSqlDatabase::addDatabase("QSQLITE");
    mojabaza.setDatabaseName(pot);
    QFileInfo preveri (pot);
    if (preveri.isFile())
    {if (mojabaza.open()) ui->status->setText("[+]Povezava z bazo je uspela!");
    }
    else
    ui->status->setText("[!]Povezava z bazo NI uspela!");
    this->model1=new QSqlQueryModel();
 
}
 
MainWindow::~MainWindow()
{
    delete ui;
    mojabaza.close();     ///?? should throw the error here
}

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

    mojabaza=QSqlDatabase::addDatabase("QSQLITE");
    mojabaza.setDatabaseName(pot);
    QFileInfo preveri (pot);
    if (preveri.isFile())
    {if (mojabaza.open()) ui->status->setText("[+]Povezava z bazo je uspela!");
    }
    else
    ui->status->setText("[!]Povezava z bazo NI uspela!");
    this->model1=new QSqlQueryModel();

}

MainWindow::~MainWindow()
{
    delete ui;
    mojabaza.close();     ///?? should throw the error here
}

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[anda_skoa]

Why do you expect this to trigger an error in the destructor when the constructor does not?
It is either a class member, so accessible from all methods of the class, or it is not, in which case it is undeclared everywhere.

Cheers,
_

[Emit]

I figured out why it doesn't throw the error. Because it is defined in .h file like you said it is a class member.

---

Added after 1 24 minutes:

I have another question. Does function bellow bellow returns the same value?
If I leave code on line 3 in my program the program doesn't work if I leave code on line 4 it works ??

QString s1 = "some SQL command";
 
1) qry.exec(s1);
2) qry.exec("some SQL command");

QString s1 = "some SQL command";

1) qry.exec(s1);
2) qry.exec("some SQL command");

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[ChrisW67]

Line 3 and line 4 do the same thing, and will either work or not in the same way.

[Emit]

I don't think it is the same:

QString s1 = "xyz";

qDebug() << s1;
qDebug() << "xyz";

qDebug() << s1;
qDebug() << "xyz";

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Output

"xyz"
xyz

"xyz"
xyz

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[ChrisW67]

The way a QString streams itself to QDebug object is different to the way a const char* is handled. This has nothing to with your question about QSqlQuery::exec().

[Emit]

I thought there is a difference if you pass a const char* or a QString to QSqlQuery::exec().

[anda_skoa]

I thought there is a difference if you pass a const char* or a QString to QSqlQuery::exec().

QSqlQuery::exec() takes a QString argument. You always pass a QString.
Either an explicitly constructed one or an implicitly converted one.

Cheers,
_

[ChrisW67]

To be clear, when you compile this:

QSqlQuery qry;
bool ok = qry.exec("select count(*) from cpp_tutorials");

QSqlQuery qry;
bool ok = qry.exec("select count(*) from cpp_tutorials");

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the compiler looks for a function matching the arguments provided. There is no QSqlQuery::exec(const char*) so the compiler looks for an exec() accepting a single argument that can be constructed from a const char*. QSqlQuery::exec(const QString&) is the only option so the compiler looks for, and uses, the QString conversion constructor to convert a const char* to QString. What actually gets compiled is equivalent to:

QSqlQuery qry;
bool ok = qry.exec(QString("select count(*) from cpp_tutorials"));

QSqlQuery qry;
bool ok = qry.exec(QString("select count(*) from cpp_tutorials"));

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This is C++ behaviour and not specific to Qt.

[Emit]

I want to add MySQL commands to the text browser in a way shown bellow. Is there a way to add time and date as well as well?

ui->textBrowser_MySQL_log->append("SELECT * FROM learn_table ");

ui->textBrowser_MySQL_log->append("SELECT * FROM learn_table ");

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[ChrisW67]

You can add any text you like.

[Emit]

I found a way already.
In case somebody will need it.

ui->textBrowser_MySQL_log->append(QDateTime::currentDateTime().toString() + ": "+ "SELECT * FROM learn_table ");

ui->textBrowser_MySQL_log->append(QDateTime::currentDateTime().toString() + ": "+ "SELECT * FROM learn_table ");

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[Emit]

I found that it won't display

": "

": "

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this part of the string. What is the reason for it?

---

Added after 14 minutes:

ui->textBrowser_MySQL_log->append(QDateTime::currentDateTime().toString() + ": "+ "SELECT * FROM learn_table ");
qDebug() << ui->textBrowser_MySQL_log->toPlainText();

ui->textBrowser_MySQL_log->append(QDateTime::currentDateTime().toString() + ": "+ "SELECT * FROM learn_table ");
qDebug() << ui->textBrowser_MySQL_log->toPlainText();

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Why doesn't the second line of the code prints text form the text browser?

[Emit]

I have Qt Widget application and inside I have a function:

void MainWindow::on_pushButton_pull_DB_clicked()

void MainWindow::on_pushButton_pull_DB_clicked()

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inside of that function I initialize a pointer called model1. Why can't I delete the pointer at the end of that function with delete model1? If I use the command my program won't work.

void MainWindow::on_pushButton_pull_DB_clicked()
{
     model1 = new QSqlQueryModel();
 
    if (db.isOpen()){
 
        model1->setQuery("SELECT * FROM learn_table");
        ui->textBrowser_MySQL_log->append(QDateTime::currentDateTime().toString() + ": "+ "SELECT * FROM learn_table ");
 
        if (model1->lastError().isValid()){
            qDebug() << "Error_100: " << model1->lastError();       //  isValid()?
        }
    }
    else {
        qDebug() << "Db is not opened, Error_101";
    }
 
   ui->tableView->setModel(model1);
 
//   delete model1; // If I uncomment this line  the program will not work
 
}

void MainWindow::on_pushButton_pull_DB_clicked()
{
     model1 = new QSqlQueryModel();

    if (db.isOpen()){

        model1->setQuery("SELECT * FROM learn_table");
        ui->textBrowser_MySQL_log->append(QDateTime::currentDateTime().toString() + ": "+ "SELECT * FROM learn_table ");

        if (model1->lastError().isValid()){
            qDebug() << "Error_100: " << model1->lastError();       //  isValid()?
        }
    }
    else {
        qDebug() << "Db is not opened, Error_101";
    }

   ui->tableView->setModel(model1);

//   delete model1; // If I uncomment this line  the program will not work

}

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[anda_skoa]

look at the line before.

Cheers,
_

[Emit]

where should i putt the delete statement than?

[ChrisW67]

You need to consider the lifetime of the model object. Create it when it needs to start existing and delete it when it needs to be disposed of. You cannot display the data from a model if you have deleted it.

[Emit]

I placed it in the destructor but the program crashes.

[Emit]

I wanted to write a simple program that connects to MySQL database and shows the name of the tables in a database.
Somehow I wasn't successful. Anybody knows why?

#include <QCoreApplication>
#include <QtSql>
#include <Qdebug>
#include <iostream>
#include <QString>
#include <QStringList>
 
using namespace std;
 
int main()
{
    QString string1;
 
 
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
    db.setHostName("localhost");
    db.setDatabaseName("cpp_delavnica");
    db.setUserName("root");
    db.setPassword("foo.bar");
 
//   QStringList lst = db.tables(); 
 
//   foreach(QString itm, lst){
 
//       qDebug() << itm;
//   }
 
 
 
    return 0;
}

#include <QCoreApplication>
#include <QtSql>
#include <Qdebug>
#include <iostream>
#include <QString>
#include <QStringList>

using namespace std;

int main()
{
    QString string1;


    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
    db.setHostName("localhost");
    db.setDatabaseName("cpp_delavnica");
    db.setUserName("root");
    db.setPassword("foo.bar");

//   QStringList lst = db.tables(); 

//   foreach(QString itm, lst){

//       qDebug() << itm;
//   }



    return 0;
}

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---

Added after 16 minutes:

i figured out what is wrong. Here is the working code if somebody else will need it.

#include <QCoreApplication>
    #include <QtSql>
    #include <Qdebug>
    #include <iostream>
    #include <QString>
    #include <QStringList>
 
    using namespace std;
 
    int main()
    {
    QString string1;
 
 
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
    db.setHostName("localhost");
    db.setDatabaseName("cpp_delavnica");
    db.setUserName("root");
    db.setPassword("foo.bar");
 
    db.open();
 
     QStringList lst = db.tables();
 
     foreach(QString itm, lst){
 
     qDebug() << itm;
     }
 
 
 
    return 0;
    }

#include <QCoreApplication>
    #include <QtSql>
    #include <Qdebug>
    #include <iostream>
    #include <QString>
    #include <QStringList>
     
    using namespace std;
     
    int main()
    {
    QString string1;
     
     
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
    db.setHostName("localhost");
    db.setDatabaseName("cpp_delavnica");
    db.setUserName("root");
    db.setPassword("foo.bar");
     
    db.open();

     QStringList lst = db.tables();
     
     foreach(QString itm, lst){
     
     qDebug() << itm;
     }
     
     
     
    return 0;
    }

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[anda_skoa]

I placed it in the destructor but the program crashes.

Did you unset the model on the view before deleting it or at least deleted the view before deleting the model?

Cheers,
_