Integrating QML and C++
QML applications often need to handle more advanced and performance-intensive tasks in C++. The most common and quickest way to do this is to expose the C++ class to the QML runtime, provided the C++ implementation is derived from QObject. Assuming that you have Qt 5.7 or later installed, the following step-by-step instructions guide you through the process of using the C++ class, BackEnd, in a QML application:
- Create a new project using the "Qt Quick Application" template in Qt Creator
Note: Uncheck the With ui.qml file option in the Define Project Details section of New Project Wizard.
- Add a new C++ class called
BackEndto the project and replace its header file contents with:#ifndef BACKEND_H #define BACKEND_H #include <QObject> #include <QString> #include <qqml.h> class BackEnd : public QObject { Q_OBJECT Q_PROPERTY(QString userName READ userName WRITE setUserName NOTIFY userNameChanged) QML_ELEMENT public: explicit BackEnd(QObject *parent = nullptr); QString userName(); void setUserName(const QString &userName); signals: void userNameChanged(); private: QString m_userName; }; #endif // BACKEND_H
The
Q_PROPERTYmacro declares a property that could be accessed from QML. TheQML_ELEMENTmacro makes the BackEnd class available in QML. - Add the following lines to your project file:
CONFIG += qmltypes QML_IMPORT_NAME = io.qt.examples.backend QML_IMPORT_MAJOR_VERSION = 1
The BackEnd class is automatically registered as a type, which is accessible from QML by importing the URL, "
io.qt.examples.backend 1.0". - Replace the contents of
backend.cppwith:#include "backend.h" BackEnd::BackEnd(QObject *parent) : QObject(parent) { } QString BackEnd::userName() { return m_userName; } void BackEnd::setUserName(const QString &userName) { if (userName == m_userName) return; m_userName = userName; emit userNameChanged(); }
The
setUserNamefunction emits theuserNameChangedsignal every timem_userNamevalue changes. The signal can be handled from QML using theonUserNameChangedhandler. - Replace the contents of
main.qmlwith the following code:import QtQuick 2.6 import QtQuick.Controls 2.0 import io.qt.examples.backend 1.0 ApplicationWindow { id: root width: 300 height: 480 visible: true BackEnd { id: backend } TextField { text: backend.userName placeholderText: qsTr("User name") anchors.centerIn: parent onTextChanged: backend.userName = text } }
The
BackEndinstance lets you access theuserNameproperty, which is updated when the TextField'stextproperty changes.
Now the application can be run.

Application running on Ubuntu
Qt offers several methods to integrate C++ with QML, and the method discussed in this tutorial is just one of them. For more details about these methods, refer to Overview - QML and C++ Integration.