My fix for the strange behaviour of QValidator.

[ntos]

I was going to ask a question about QIntValidator strange behaviour of allowing you to enter a number larger than the max value when I knew I had to search for an answer on this forum first. No solution could be found. Anyway, with the help of chatgpt I have come up with a solution. people here have helped me so I think it may help beginners like me to post the solution here:
P.S. If this is not appropriate, please delete this post. Thank you very much.

#include "MainWindow.h"
#include "ui_MainWindow.h"
#include <QIntValidator>
#include <QDoubleValidator>
#include <QLineEdit>
#include <QGridLayout>
#include <QLabel>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    this->setGeometry(50, 50, 120, 50);
    QGridLayout* layout = new QGridLayout();
    QLineEdit* intEdit = new QLineEdit("0");
    QLineEdit* doubleEdit = new QLineEdit("0.0");
    layout->addWidget(new QLabel("Enter an integer: "), 0, 0);
    // second pair: 1, 2 means row occupies 1 row, 2 means col occupies 2 cols.
    layout->addWidget(intEdit, 0, 1, 1, 2);
    layout->addWidget(new QLabel("Enter a double: "), 1, 0);
    layout->addWidget(doubleEdit, 1, 1, 1, 2);

    QIntValidator* intValidator = new QIntValidator(-50, 100);
   // intValidator->setRange(0, 100); // we can use this.
    intEdit->setValidator(intValidator);
    QString* previousValidValue = new QString(intEdit->text());

    // NOTE that we use the textChanged SIGNAL so that we can check each single input character in real time.
    connect(intEdit, &QLineEdit::textChanged, [intEdit, intValidator, previousValidValue] {
        QString valueStr = intEdit->text();
        int pos = 0; // Dummy variable needed by validate, but unused.

    // Note that only this IF CONDITION works. Nothing else will not work as expected.
    // A. if valueStr is not within bounds ( -50 =< valueStr <= 100), if returns true. So we call setText(previous_value) on intEdit.
    // To make this clear, suppose we enter 9, which is valid so the else clause runs and previousValidValue == 9.
    // We enter 9 again and intEdit now has 99 and valueStr = 99, valid and previousValidValue == 99. When we enter a third digit,
    // now intEdit contains 990 and if returns true and intEdit->setText(*previousValidValue); runs, which displays 99.
    // That's why we can never exceed 100.
    // B. if valueStr is within bounds, if returns false. So we update previousValidValue with the current valid input.
    // C. If we comment out the else clause, and enter a two-digit number more than once, we'll get 0 + the two-digit number.
    // 0 is our initial number given to the constructor of intEdit.
    // D. We cannot enter 1-0, i.e the minus sign after a digit. Great!
        if (intValidator->validate(valueStr, pos) != QValidator::Acceptable
                    && !valueStr.isEmpty()
                    && valueStr != "-")
        {
            intEdit->setText(*previousValidValue);
        } else {
            // Update previous valid value if current input is valid
            *previousValidValue = valueStr;
        }
    });
       // add layout to the central widget to be displayed.
    this->centralWidget()->setLayout(layout);

}

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