QLineEdit does not clear its `text()` after the clear button is clicked

Robert Hairgrove

Using Qt 5.15.3 on Ubuntu 24.04.3:

I have a line edit widget with the 'clear' button enabled. I can connect my code to its signal (QAction::triggered(), there is some code on this forum which shows how to do it) which forwards to another function which is a slot connected to the editingFinished() signal of the line edit.

In the slot connected to editingFinished(), I query the line edit's text() method -- this should return an empty string when the clear button is clicked; however, it shows me the old content, although it is no longer visible in the line edit widget.

The code works fine when the slot is called normally by tabbing out of the line edit widget.

Is this a known bug?

Christian Ehrlicher

@Robert-Hairgrove said in QLineEdit does not clear its `text()` after the clear button is clicked:

Is this a known bug?

Yes, it's known as a programmers error.
you are doing something wrong. Please provide a minimal, compileable example of the problem.

Robert Hairgrove

@Christian-Ehrlicher Very funny.

Here is some example code -- a screenshot was taken and attached at the end after the code.
Built with Qt 6.9.1 and gcc 13 on Ubuntu 24.04.3 LTS.

NOTE: The slot which is connected to the clear button's QAction calls "handleEditingFinished()", which is where it would seem to be expected that calling "QLineEdit::text()" should return an empty string. However, it only returns the empty string in the "textChanged()" slot.

Project file (qmake): "Test_LineEdit.pro"

QT += core gui widgets
CONFIG += c++17
SOURCES += \
    main.cpp \
    MainWindow.cpp
HEADERS += \
    MainWindow.hpp
FORMS += \
    MainWindow.ui

main.cpp:

#include "MainWindow.hpp"
#include <QApplication>
int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  MainWindow w;
  w.show();
  return a.exec();
}

MainWindow.hpp

#ifndef MAINWINDOW_HPP
#define MAINWINDOW_HPP

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui {
class MainWindow;
}
QT_END_NAMESPACE

class MainWindow : public QMainWindow {
  Q_OBJECT
public:
  MainWindow(QWidget *parent = nullptr);
  ~MainWindow();
public slots:
  void handleBrowseClicked(bool);
  void handleClearButtonClicked(bool);
  void handleEditingFinished();
  void handleTextChanged(const QString &txt);
private:
  Ui::MainWindow *ui;
};
#endif // MAINWINDOW_HPP

MainWindow.cpp:

#include <QtCore>
#include <QFileDialog>
#include <QtDebug>
#include "MainWindow.hpp"
#include "ui_MainWindow.h"

MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent)
  , ui(new Ui::MainWindow)
{
  ui->setupUi(this);
  // Connect signals to slots:
  connect(ui->btnBrowse, &QPushButton::clicked,
          this, &MainWindow::handleBrowseClicked);
  connect(ui->edtInput, &QLineEdit::editingFinished,
          this, &MainWindow::handleEditingFinished);
  connect(ui->edtInput, &QLineEdit::textChanged,
          this, &MainWindow::handleTextChanged);
  QList<QAction*> actionList
      = ui->edtInput->findChildren<QAction*>();

  // Connects the 'clear' button's QAction to our slot:
  if (!actionList.isEmpty()) {
    connect(actionList.first(), &QAction::triggered,
            this, &MainWindow::handleClearButtonClicked);
  }
}

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

void MainWindow::handleBrowseClicked(bool)
{
  static const QString msg(__func__);
  qDebug() << msg;
  ui->txtLog->appendPlainText(msg);

  const QString fname = QFileDialog::getOpenFileName(
      this,"Select Any File");
  if (!fname.isEmpty()) {
    ui->edtInput->setText(fname);
  }
}

void MainWindow::handleClearButtonClicked(bool)
{
  static const QString msg(__func__);
  qDebug() << msg;
  ui->txtLog->appendPlainText(msg);

  handleEditingFinished();
}

void MainWindow::handleEditingFinished()
{
  QString msg;
  msg = QString(__func__).append("\n\tInput: ").append(ui->edtInput->text());
  qDebug() << msg;
  ui->txtLog->appendPlainText(msg);
}

void MainWindow::handleTextChanged(const QString &txt)
{
  QString msg;
  msg = QString(__func__).append("\n\tInput: ").append(txt);
  qDebug() << msg;
  ui->txtLog->appendPlainText(msg);
}

MainWindow.ui:

<?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>669</width>
    <height>329</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <layout class="QGridLayout" name="gridLayout">
    <item row="0" column="0">
     <layout class="QVBoxLayout" name="verticalLayout_2">
      <item>
       <layout class="QHBoxLayout" name="horizontalLayout">
        <item>
         <widget class="QLabel" name="lblInput">
          <property name="text">
           <string>&amp;File name:</string>
          </property>
          <property name="buddy">
           <cstring>edtInput</cstring>
          </property>
         </widget>
        </item>
        <item>
         <widget class="QLineEdit" name="edtInput">
          <property name="clearButtonEnabled">
           <bool>true</bool>
          </property>
         </widget>
        </item>
        <item>
         <widget class="QPushButton" name="btnBrowse">
          <property name="text">
           <string>&amp;Browse...</string>
          </property>
         </widget>
        </item>
       </layout>
      </item>
      <item>
       <layout class="QVBoxLayout" name="verticalLayout">
        <item>
         <widget class="QLabel" name="lblLog">
          <property name="text">
           <string>&amp;Log:</string>
          </property>
          <property name="buddy">
           <cstring>txtLog</cstring>
          </property>
         </widget>
        </item>
        <item>
         <widget class="QPlainTextEdit" name="txtLog"/>
        </item>
       </layout>
      </item>
     </layout>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>669</width>
     <height>23</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <resources/>
 <connections/>
</ui>

Screenshot:

Robert Hairgrove

I think the problem is that the "clear" button is really only meant to be a convenience for editing, and that one shouldn't rely on this QAction for anything. There is some heavy lifting done only when the editing is finished, though, which I do not want to call every time the text changes.

JonB

@Robert-Hairgrove
Sorry, but I am not seeing/understanding where you say your clear button (clearButtonEnabled) is clearing your QLineEdit at all, which you seem to say it does? I have not tested it but I do not think the button itself clears the text, you have to connect something to do that? And you do not call clear() or setText() to do so? Or are you saying it does clear, but apparently not till after the QAction::triggered() has fired?

Robert Hairgrove

@JonB said in QLineEdit does not clear its `text()` after the clear button is clicked:

Or are you saying it does clear, but apparently not till after the QAction::triggered() has fired?

This is what I meant.

I think I will just set some kind of Boolean flag when the clear button was clicked and check the state of that flag in the handleEditingFinished() slot. In this case it is safer than relying on the text() function, I think.

Another idea would be to check the value returned by sender() in the handleEditingFinished() slot. If it is empty, or not from the QLineEdit widget, then it must be from calling it directly from the other slot.

IgKh

@Robert-Hairgrove said in QLineEdit does not clear its `text()` after the clear button is clicked:

QList<QAction*> actionList
      = ui->edtInput->findChildren<QAction*>();

  // Connects the 'clear' button's QAction to our slot:
  if (!actionList.isEmpty()) {
    connect(actionList.first(), &QAction::triggered,
            this, &MainWindow::handleClearButtonClicked);
  }

The moment you are doing something like this, you are making assumptions about undocumented and non-guaranteed internal implementation details of the QLineEdit. I.e that the clear button added by setClearButtonEnabled is even tied to a QAction in the object ownership hierarchy of the QLineEdit. Even if something works like this today, even a patch version of Qt can change it (they usually don't, but minor bug fixes can and do have subtle ripple effects on undocumented behavior).

In this case, the issue probably stems from the fact that both you and QLineEdit internals have connected a slot to the same signal, and you have no control over which is going to be called first. Yours was probably first, so the text is not yet cleared.

If you need fine grained control, don't use the built-in action, create one of your own using the documented API which you can manage at will.

JonB

@Robert-Hairgrove
And that may be fine.
You could also --- and it would tell you what is going on even if you don't stick with it --- make the button call handleClearButtonClicked() delayed on a QTimer::singleShot() of a few milliseconds. If that works it tells you about when the text actually gets cleared. It may mean handleClearButtonClicked() is called later than editingFinished() now, I don't know if that matters to you.

IgKh

@Robert-Hairgrove said in QLineEdit does not clear its `text()` after the clear button is clicked:

Another idea would be to check the value returned by sender() in the handleEditingFinished() slot. If it is empty, or not from the QLineEdit widget, then it must be from calling it directly from the other slot.

Few notes on that - the first is that it won't work that way. QObject::sender() isn't magic, and has no way of knowing that you called it from a function that is not directly the slot. The second is that you never know who the actual sending object is, and can't assume that it is the QLineEdit directly (and not a sub-object of it, or something else entirely).

And in general - and for anyone else reading - this can't be emphasized enough: The QObject protected methods like sender(), receivers(), connectNotify() and similar are blunt tools for very very specific situations that rarely truly arise in applications. Their use is almost certainly a code smell - and indicates that the design needs to be changed to reduce coupling.

JonB

@IgKh said in QLineEdit does not clear its `text()` after the clear button is clicked:

Few notes on that - the first is that it won't work that way. QObject::sender() isn't magic, and has no way of knowing that you called it from a function that is not directly the slot. The second is that you never know who the actual sending object is, and can't assume that it is the QLineEdit directly (and not a sub-object of it, or something else entirely).

@Robert-Hairgrove said in QLineEdit does not clear its `text()` after the clear button is clicked:

Another idea would be to check the value returned by sender() in the handleEditingFinished() slot. If it is empty, or not from the QLineEdit widget, then it must be from calling it directly from the other slot.

Set the connect() on editingFinished to call a lambda, passing on ui->edtInput explicitly to your slot rather than trying to use sender().