**Course Title:** Qt 6 Application Development with C++ **Section Title:** Advanced Widgets and Custom Forms **Topic:** Customizing input forms with validators (e.g., QValidator for QLineEdit). **Introduction** In Qt 6 application development, validating user input is crucial to ensure data consistency and prevent errors. Qt provides a powerful mechanism for input validation through the `QValidator` class, which can be used with various input widgets, such as `QLineEdit`. In this topic, we will explore how to customize input forms using validators, specifically focusing on `QValidator` for `QLineEdit`. **Understanding QValidator** `QValidator` is an abstract base class that provides a way to validate input data. It defines a set of functions that must be reimplemented by derived classes to provide validation logic. The most commonly used functions are: * `void fixup(QString &input)`: Attempts to fix the input data to match the expected format. * `State validate(QString &input, int &position)`: Returns the validation state (Invalid, Intermediate, or Acceptable) based on the input data. * `QValidator::State state()`: Returns the current validation state. Qt provides several built-in validators, including: * `QIntValidator`: Validates integer input. * `QDoubleValidator`: Validates double-precision floating-point numbers. * `QRegExpValidator`: Validates input data using regular expressions. **Using QValidator with QLineEdit** To use a `QValidator` with a `QLineEdit` widget, you need to create an instance of the validator and then set it using the `setValidator()` function. Here is an example: ```cpp #include <QLineEdit> #include <QIntValidator> QLineEdit *lineEdit = new QLineEdit(this); QIntValidator *validator = new QIntValidator(0, 100, this); lineEdit->setValidator(validator); ``` In this example, we create a `QIntValidator` that validates integers between 0 and 100. We then set this validator for the `QLineEdit` widget using the `setValidator()` function. **Creating a Custom Validator** While Qt provides several built-in validators, there may be cases where you need a custom validator. To create a custom validator, you need to derive a class from `QValidator` and reimplement the `validate()` function. Here is an example of a custom validator that checks if the input is a valid email address: ```cpp #include <QValidator> #include <QRegExp> class EmailValidator : public QValidator { public: EmailValidator(QObject *parent = nullptr) : QValidator(parent) {} State validate(QString &input, int &position) const override { QRegExp emailRegex("[^@]+@[^@]+"); if (emailRegex.exactMatch(input)) { return Acceptable; } else { return Invalid; } } }; ``` In this example, we create a custom validator called `EmailValidator` that checks if the input matches a regular expression for a valid email address. If the input matches, the `validate()` function returns `Acceptable`; otherwise, it returns `Invalid`. **Example Use Case** Here is an example of using the custom `EmailValidator` with a `QLineEdit` widget: ```cpp QLineEdit *lineEdit = new QLineEdit(this); EmailValidator *validator = new EmailValidator(this); lineEdit->setValidator(validator); ``` **Conclusion** In this topic, we explored how to customize input forms using validators in Qt 6 application development. We discussed the `QValidator` class and how to use it with `QLineEdit` widgets. We also created a custom validator to validate email addresses. **Key Takeaways** * Use `QValidator` to validate input data in Qt 6 applications. * Derive a class from `QValidator` to create custom validators. * Use built-in validators, such as `QIntValidator` and `QRegExpValidator`, when possible. **External Resources** * [Qt Documentation: QValidator](https://doc.qt.io/qt-6/qvalidator.html) * [Qt Documentation: QLineEdit](https://doc.qt.io/qt-6/qlineedit.html) **What's Next** In the next topic, we will cover 'Creating reusable custom widgets using inheritance and composition.' If you have any questions or need further clarification on this topic, please feel free to comment below.