QIntValidator

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You can prevent invalid text from being entered into a lineEdit using a validator. This example demonstrates QIntValidator, which only allows integers to be entered into the lineEdit.

Validator.cpp

#include <QApplication>

#include "form.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    Form form;
    form.show();
    return app.exec();
}

form.h

#ifndef FORM_H
#define FORM_H

#include "ui_form.h"

class Form : public QWidget, private Ui::Form
{
    Q_OBJECT
public:
    Form(QWidget* parent=0);
public slots:
    void on_lineEdit1_textEdited( const QString & text );
    void on_lineEdit2_textEdited( const QString & text );
};

#endif

form.cpp

#include <QtGui>
#include <iostream>

#include "form.h"

Form::Form(QWidget *parent)
    : QWidget(parent)
{
    setupUi(this);
    // Limit input to valid values between 0 and 255. Either parent ('this' or 'lineEdit') works, I'm not sure what difference it makes.
    QIntValidator* validator =
    //new QIntValidator(0, 255, this->lineEdit);
        new QIntValidator(0, 255, this);
    this->lineEdit1->setValidator(validator);
    this->lineEdit2->setValidator(validator);
}

void Form::on_lineEdit1_textEdited(const QString& text)
{
    std::cout << "text1: " << text.toStdString() << std::endl;
    std::cout << "number1: " << text.toUInt() << std::endl;
}

void Form::on_lineEdit2_textEdited(const QString& text)
{
    std::cout << "text2: " << text.toStdString() << std::endl;
}

form.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Form</class>
 <widget class="QWidget" name="Form">
 <property name="geometry">
 <rect>
 <x>0</x>
 <y>0</y>
 <width>400</width>
 <height>300</height>
 </rect>
 </property>
 <property name="windowTitle">
 <string>Form</string>
 </property>
 <widget class="QLineEdit" name="lineEdit1">
 <property name="geometry">
 <rect>
 <x>250</x>
 <y>90</y>
 <width>113</width>
 <height>27</height>
 </rect>
 </property>
 </widget>
 <widget class="QLineEdit" name="lineEdit2">
 <property name="geometry">
 <rect>
 <x>250</x>
 <y>170</y>
 <width>113</width>
 <height>27</height>
 </rect>
 </property>
 </widget>
 </widget>
 <resources/>
 <connections/>
</ui>