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 &amp; text )
{
 std::cout << "text1: " << text.toStdString() << std::endl;
 std::cout << "number1: " << text.toUInt() << std::endl;
}

void Form::on_lineEdit2_textEdited ( const QString &amp; 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>