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Qt Framework Cheatsheet

A quick reference guide to the Qt framework, covering essential classes, functions, and concepts for developing cross-platform applications.

Core Concepts

Signals and Slots

Qt’s signal and slot mechanism facilitates communication between objects. A signal is emitted when a particular event occurs, and a slot is a function that is called in response to a signal.

  • signals: keyword declares signals within a class.
  • slots: keyword declares slots within a class.
  • connect() function establishes the connection between a signal and a slot.

Example:

// Define a signal
signals:
    void buttonClicked();

// Define a slot
public slots:
    void handleButtonClicked();

// Connect the signal and slot
connect(button, &QPushButton::clicked, this, &MyClass::handleButtonClicked);

Qt’s signals and slots provide a type-safe way to implement callbacks, reducing the risk of runtime errors.

Meta-Object System

The Meta-Object System (MetaObject) provides information about the objects at runtime.

  • Q_OBJECT macro is mandatory in any class that uses signals and slots or other meta-object features.
  • QMetaObject class provides access to meta-information about a class.
  • Allows for dynamic property access and invocation of methods.

Example:

class MyClass : public QObject {
    Q_OBJECT
public:
    MyClass(QObject *parent = nullptr) : QObject(parent) {}
};

Object Model

Qt’s object model is based on a hierarchical object tree. QObject is the base class for all Qt objects that support object hierarchies, signals, and slots.

  • Parent-child relationships manage object lifetimes.
  • Deleting a parent object will also delete its children.
  • QObject::parent() returns the parent of an object.

Example:

QObject *parent = new QObject();
QObject *child = new QObject(parent);

// When parent is deleted, child is also deleted.
delete parent;

Common Classes

QWidgets

QPushButton

A button that the user can click.

QLabel

Displays text or an image.

QLineEdit

A single-line text editor.

QTextEdit

A multi-line text editor with rich text support.

QComboBox

A combo box widget (drop-down list).

QCheckBox

A checkbox widget.

QSlider

A horizontal or vertical slider.

QProgressBar

Displays the progress of a task.

Layout Managers

QVBoxLayout

Arranges widgets vertically.

QHBoxLayout

Arranges widgets horizontally.

QGridLayout

Arranges widgets in a grid.

QFormLayout

Arranges widgets in a two-column form.

Data Handling

Containers

QList<T>

A dynamically-sized array.

QVector<T>

Provides contiguous storage.

QMap<Key, Value>

A key-value storage.

QSet<T>

Stores unique values.

QStringList

A list of strings.

String Handling

QString is Qt’s string class.

  • It provides support for Unicode.
  • It is implicitly shared.
  • It offers many manipulation methods.

Common QString Methods:

  • QString::append()
  • QString::prepend()
  • QString::toLower()
  • QString::toUpper()
  • QString::trimmed()
  • QString::split()

Networking

Network Classes

QTcpSocket

Provides a TCP socket.

QTcpServer

Listens for incoming TCP connections.

QUdpSocket

Provides a UDP socket.

QNetworkRequest

Represents a network request.

QNetworkAccessManager

Manages network requests.

HTTP Operations

Qt simplifies HTTP operations using QNetworkAccessManager and related classes.

  • get()
  • post()
  • put()
  • deleteResource()

Example:

QNetworkAccessManager *manager = new QNetworkAccessManager(this);
connect(manager, &QNetworkAccessManager::finished, this, [=](QNetworkReply *reply) {
    if (reply->error()) {
        qDebug() << "Error:" << reply->errorString();
    } else {
        qDebug() << reply->readAll();
    }
    reply->deleteLater();
});

manager->get(QNetworkRequest(QUrl("http://example.com")));
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