How to update a window in QT?

Question

Im working on a simple project in QtCreator where you input text into a line_edit which then gets printed after clicking a button. It works but I need to resize the window in order to see the updated/changed display.

So starting off with the main.cpp, I have left it as default after some tests:

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

That has the issue I was talking about above. I decided to add w.update(); and see if that fixed the issue, it did not. I thought maybe it was because the program was not looping, so I entered the code in a while(true) loop which also was to no avail.

The mainwindow.cpp file is as follows:

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

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->textBtn, SIGNAL(clicked(bool)), this, SLOT(setText()));
}

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

void MainWindow::setText()
{
    QString temp = ui->inputText->text();
    ui->displayLabel->setText(temp);

}

MainWindow.hpp:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

public slots:
    void setText();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

Is there a QObject or predifined function in QT that allows me to update the window or automatically updates the window after a detected user change?

Edit: The UI file might be of importance as well:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>554</width>
    <height>463</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <widget class="QLabel" name="displayLabel">
    <property name="geometry">
     <rect>
      <x>140</x>
      <y>150</y>
      <width>251</width>
      <height>91</height>
     </rect>
    </property>
    <property name="text">
     <string/>
    </property>
    <property name="scaledContents">
     <bool>true</bool>
    </property>
    <property name="wordWrap">
     <bool>true</bool>
    </property>
   </widget>
   <widget class="QWidget" name="layoutWidget">
    <property name="geometry">
     <rect>
      <x>130</x>
      <y>30</y>
      <width>251</width>
      <height>81</height>
     </rect>
    </property>
    <layout class="QVBoxLayout" name="verticalLayout">
     <item>
      <widget class="QLineEdit" name="inputText"/>
     </item>
     <item>
      <widget class="QPushButton" name="textBtn">
       <property name="text">
        <string>Display Text</string>
       </property>
      </widget>
     </item>
    </layout>
   </widget>
  </widget>
  <widget class="QMenuBar" name="menuBar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>554</width>
     <height>22</height>
    </rect>
   </property>
  </widget>
  <widget class="QToolBar" name="mainToolBar">
   <attribute name="toolBarArea">
    <enum>TopToolBarArea</enum>
   </attribute>
   <attribute name="toolBarBreak">
    <bool>false</bool>
   </attribute>
  </widget>
  <widget class="QStatusBar" name="statusBar"/>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

Answer 1

The problem is not the update of the GUI but the QLabel does not change size, the initial size depends on the initial text, and if you set a text with larger size only part of the text will be displayed. To adjust the size of the label to the size of the text you must use adjustSize():

void MainWindow::setText()
{
    QString temp = ui->inputText->text();
    ui->displayLabel->setText(temp);
    ui->displayLabel->adjustSize();
}

On the other hand in Qt5 it is advisable to use the new connection syntax since they have several advantages as indicated by the docs, in your case you must change your code to:

connect(ui->textBtn, &QPushButton::clicked, this, &MainWindow::setText);