[SOLVED] What am I doing wrong trying to change QPushButton text at runtime?

Issue

I’m new to coding with QTCreator, and Linux Graphical Development in general. What is the, probably trivial, thing I am doing wrong here?

I could write this code and make it work easily without using the form method, but I’d really prefer to use the graphical interface, as it should be faster (once I know what compiler wants me to do).

When I enter this code it tells me that btnStopGo isn’t defined in this context.
I used the forms interface to drop the button onto a form.

The code is in the file mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

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

...

void MainWindow::on_btnStopGo_clicked()
{
    if (bDoStream){
        if (bStreaming){
            //TODO: Send stop stream signal, save stream to file, dispose stream
            bStreaming = false;
            btnStopGo->setText("Go");
        }
        else{
            btnStopGo->setText("Stop");
            bStreaming = true;
            // TODO: request a stream of quotes
        }
    }
    else {
        // TODO: Get a single quote tick
    }

}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

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_btnSelect_clicked();

    void on_btnStopGo_clicked();

private:
    Ui::MainWindow *ui;

};
#endif // MAINWINDOW_H

Other visible files in the project are : Hello2.pro, main.cpp, and mainwindow.ui. main.cpp declares mywindow as the variable to access the main window. I’ve tried using "ui.btnStopGo", mywindow.btnStopGo, ui->btnStopGo (blows up the compiler), and every other combo I can think of. None of them work so far.

I don’t know if this is relevant but I’m using:
Qt Creator 4.11.0
Based on Qt 5.12.8 (GCC 9.3.0, 64 bit)
on a Linux Mint 20 Cinnamon with KDE added manually.

Solution

If you put the button in the form designer, and called the button btnStopGo, then it ends up compiled into the ui_mainwindow.h and .cc files. Your MainWindow class, which you wrote yourself, doesn’t have a (member) variable called btnStopGo. It does have a variable ui, which points to the user interface which was generated from the form designer.

Use ui->btnStopGo->setText("stop") instead.

Answered By – Adriaan de Groot

Answer Checked By – Candace Johnson (BugsFixing Volunteer)