Can a Qwindow have plain c++ attribute ?

Louce

Hi there.

As far as i search it seems that I don't have a regular behavior on Qt.

I'm trying to play with attribute in a window. At the moment a simple int.

I have a int i in my window attribute and I set it inside the constructor.
but I soon as i leave the constructor my int seems to have a random value

main.cpp

 int i = 1;
    QApplication a(argc, argv);
    MainMenuWindow w = new MainMenuWindow(i);
    w.show();

Window.h

   MainMenuWindow(int, QWidget *parent = nullptr);

    void setI(int i);

    int getI();
private slots:

    void on_pushButton_3_clicked();

private:

    Ui::MainMenuWindow *ui;
    int i;

Window.cpp

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

    this->i = i;
  
    std::cout << "Test in constructor" << std::endl;
    std::cout << this->i << std::endl;
}

void MainMenuWindow::on_pushButton_3_clicked()
{
    std::cout << "test with getI" << std::endl;
    std::cout << this->getI() << std::endl;
    std::cout << "test with attribute" << std::endl;
    std::cout << this->i << std::endl;
}

And when I start the app and click on the push buton 3 :

I'm a bit confused here as it seems to behave like this with other attributes types.

Hope you can light me on this.

Good day to you all.

Christian Ehrlicher

You don't show what getI() does.
Also please format your code so it's readable - please use the code tags.

JonB

@Louce
Also where do the two QT App start messages come from?

Louce

Sorry to both of you I did use to code tag but i removed a lot of irrelevant code from my file to be cleaner for you.

I'll try again

Here's the main.cpp

#include "mainmenuwindow.h"
#include "iostream"

#include <QApplication>

int main(int argc, char *argv[])
{

    int i = 1;
    QApplication a(argc, argv);
    MainMenuWindow w = new MainMenuWindow(i);
    w.show();
    return a.exec();
}

Here's the MainMenuWindow.h

#ifndef MAINMENUWINDOW_H
#define MAINMENUWINDOW_H

#include <QMainWindow>
#include <QLabel>

QT_BEGIN_NAMESPACE
namespace Ui { class MainMenuWindow; }
QT_END_NAMESPACE

class MainMenuWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainMenuWindow(QWidget *parent = nullptr);

    MainMenuWindow(int, QWidget *parent = nullptr);

    ~MainMenuWindow();

    void setI(int i);

    int getI();
private slots:
    void on_pushButton_3_clicked();

private:

    Ui::MainMenuWindow *ui;
    int i;
};
#endif // MAINMENUWINDOW_H

Here's the MainMenuWindow.cpp

#include "mainmenuwindow.h"

#include "ui_mainmenuwindow.h"

#include <string>
#include <iostream>

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

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

    this->i = i;
    std::cout << "Test in constructor" << std::endl;
    std::cout << this->i << std::endl;

}

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

void MainMenuWindow::setI(int i) {
    this->i = i;
}

int MainMenuWindow::getI() {
    return this->i;
}


void MainMenuWindow::on_pushButton_3_clicked()
{
    std::cout << "test with getI" << std::endl;
    std::cout << this->getI() << std::endl;
    std::cout << "test with attribute" << std::endl;
    std::cout << this->i << std::endl;
}

Result after starting the app and cliking on the button 3 ->

I have the feeling that it's more an error from my side due to my lack of Qt knowledge.

Thanks again.

SGaist

Hi,

Nothing stands out a fundamentally wrong (except the .show call as it should be a -> since it's a pointer).

One thing you should add is the const qualifier for your getter.

Louce

well .... it was just that.

with this change it's working. ;

int main(int argc, char *argv[])
{

    int i = 1;
    QApplication a(argc, argv);
    MainMenuWindow* w = new MainMenuWindow(i);
    w->show();
    return a.exec();
}

apparantly Qt compute a non pointer with " = new MainMenuWindow() and then accept the w.show but I guess he went crazy after.

Quick question, Do I have to declare it MainMenuWindow* w like above, ore the fact dans I write = new MainMenuWindow makes it automaticaly a pointer ?

Thanks anyway your help :) .

Pl45m4

@Louce said in Can a Qwindow have plain c++ attribute ?:

Quick question, Do I have to declare it MainMenuWindow* w like above, ore the fact dans I write = new MainMenuWindow makes it automaticaly a pointer ?

Using new...-"something" allocates memory on heap and returns/assigns the pointer to that memory. So you need a variable, which is a pointer like MainMenuWindow *.

MainMenuWindow w = new MainMenuWindow;
wont work.

BTW: This has nothing to do with Qt. This is how C++ is.

int i = new int(2); // WRONG
// If you dont need a ptr, use stack allocation
int i = 2;

int * j = new int(2); // This works

Louce

@Pl45m4

Thanks for the clarification.

Yes I understood that's wasn't related to Qt.

Thank a lot to all of you.