Qt passing a Value from one window to antother

Question

I am quite new to programming, so hopefully someone can help me out. After calculating a value with a function and storing it into the variable with a setter, I want to call it in my viswindow.cpp to visualize it in a Graph. In the mainwindow the right value is calculated and stored by pressing a button, which also opens the viswindow. I tried to pass it with a pointer or with signals and slots, but it wouldn't work.

mainwindow.cpp:

double MainWindow::berechneFussabdruck(double strecke, int anzahl, double sfcWert)
{
    if(strecke <= 1500 && strecke > 0){
        double aequivalente_Co2 = (((sfcWert*3.116*50*(((0.0011235955)*(strecke-2*48.42))+0.233))*0.001)/anzahl);
        setAequivalente(aequivalente_Co2);
    }
    else{
        double aequivalente_Co2 = (((sfcWert*3.116*75*(((0.0011235955)*(strecke-2*208))+0.833))*0.001)/anzahl);
        setAequivalente(aequivalente_Co2);
    }
   return aequivalente_Co2;
}

Button to open viswindow:

void MainWindow::on_pushButton_clicked()
{
    MainWindow::berechneFussabdruck(strecke, anzahl, sfcWert);
    visWindow = new VisWindow(this);
    visWindow->show();   
}

viswindow.cpp:

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

    ...

    *set0 << aequivalente_Co2 << aequivalente_Co2;
    *set1 << 11.07 << 0;
    *set2 << 0 << 0.49;
    
    ...
}

Thanks for helping!

Answer 1

Here are examples of:

1. Single time value transfer from MainWindow to VisWindow in constructor parameter.
2. Value update from MainWindow to VisWindow by slot-signal chain.
3. Value update from MainWindow to VisWindow by direct call of VisWindow->updateValue() slot as regular function.

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include "viswindow.h"
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
private slots:
    void on_pushButton_clicked();
    void on_pushButton_2_clicked();
    void on_pushButton_3_clicked();
signals:
    void updatevalue(const double value);
private:
    Ui::MainWindow *ui;
    VisWindow * m_viswindow;
};
#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow) 
{
    ui->setupUi(this);
}
MainWindow::~MainWindow() {
    delete ui;
}
void MainWindow::on_pushButton_clicked() {
    m_viswindow = new VisWindow(12.34, nullptr);
    connect(this, &MainWindow::updatevalue,
            m_viswindow, &VisWindow::updateValue);
    m_viswindow->show();
}
void MainWindow::on_pushButton_2_clicked() {
    emit updatevalue(23.45);
}
void MainWindow::on_pushButton_3_clicked() {
    m_viswindow->updateValue(45.67);
}

viswindow.h

#ifndef VISWINDOW_H
#define VISWINDOW_H
#include <QWidget>
namespace Ui {
class VisWindow;
}
class VisWindow : public QWidget
{
    Q_OBJECT
public:
    explicit VisWindow(const double value, QWidget *parent = nullptr);
    ~VisWindow();
public slots:
    void updateValue(const double value);
private:
    Ui::VisWindow *ui;
};
#endif // VISWINDOW_H

viswindow.cpp

#include "viswindow.h"
#include "ui_viswindow.h"
VisWindow::VisWindow(const double value, QWidget *parent) :
    QWidget(parent),
    ui(new Ui::VisWindow)
{
    ui->setupUi(this);
    ui->lblValue->setText(QString::number(value,'f',2));
    this->setWindowTitle("VisWindow");
}
VisWindow::~VisWindow() {
    delete ui;
}
void VisWindow::updateValue(const double value) {
    ui->lblValue->setText(QString::number(value,'f',2));
}