Home > Articles > Programming > C/C++

📄 Contents

  1. Customizing QtWidgets
  2. Subclassing QWidget
  3. Integrating Custom Widgets with QtDesigner
  4. Double Buffering
  • Print
  • + Share This
This chapter is from the book

Integrating Custom Widgets with Qt Designer

Before we can use custom widgets in Qt Designer, we must make Qt Designer aware of them. There are two techniques for doing this: the "promotion" approach and the plugin approach.

The promotion approach is the quickest and easiest. It consists of choosing a built-in Qt widget that has a similar API to the one we want our custom widget to have and filling in Qt Designer's custom widget dialog (shown in Figure 5.6) with some information about the widget. The custom widget can then be used in forms developed with Qt Designer, although it will be represented by the associated built-in Qt widget while the form is edited or previewed.


Figure 5.6 's custom widget dialog

Here's how to insert a HexSpinBox widget into a form using this approach:

  1. Create a QSpinBox by dragging it from Qt Designer's widget box onto the form.
  2. Right-click the spin box and choose Promote to Custom Widget from the context menu.
  3. Fill in the dialog that pops up with "HexSpinBox" as the class name and "hexspinbox.h" as the header file.

Voilà! The code generated by uic will include hexspinbox.h instead of <QSpinBox> and instantiate a HexSpinBox. In Qt Designer, the HexSpinBox widget will be represented by a QSpinBox, allowing us to set all the properties of a QSpinBox (e.g., the range and the current value).

The drawbacks of the promotion approach are that properties that are specific to the custom widget aren't accessible in Qt Designer and that the widget isn't rendered as itself. Both these problems can be solved by using the plugin approach.

The plugin approach requires the creation of a plugin library that Qt Designer can load at run-time and use to create instances of the widget. Qt Designer then uses the real widget when editing the form and for previewing, and thanks to Qt's meta-object system, Qt Designer can dynamically obtain the list of its properties. To show how this works, we will integrate the IconEditor from the previous section as a plugin.

First, we must subclass QDesignerCustomWidgetInterface and reimplement some virtual functions. We will assume that the plugin source code is located in a directory called iconeditorplugin and that the IconEditor source code is located in a parallel directory called iconeditor.

Here's the class definition:

#include <QDesignerCustomWidgetInterface>

class IconEditorPlugin : public QObject,
                         public QDesignerCustomWidgetInterface

    IconEditorPlugin(QObject *parent = 0);

    QString name() const;
    QString includeFile() const;
    QString group() const;
    QIcon icon() const;
    QString toolTip() const;
    QString whatsThis() const;
    bool isContainer() const;
    QWidget *createWidget(QWidget *parent);

The IconEditorPlugin subclass is a factory class that encapsulates the IconEditor widget. It is derived from both QObject and QDesignerCustomWidgetInterface and uses the Q_INTERFACES() macro to tell moc that the second base class is a plugin interface. Qt Designer uses the functions to create instances of the class and to obtain information about it.

IconEditorPlugin::IconEditorPlugin(QObject *parent)
    : QObject(parent)

The constructor is trivial.

QString IconEditorPlugin::name() const
    return "IconEditor";

The name() function returns the name of the widget provided by the plugin.

QString IconEditorPlugin::includeFile() const
    return "iconeditor.h";

The includeFile() function returns the name of the header file for the specified widget encapsulated by the plugin. The header file is included in the code generated by the uic tool.

QString IconEditorPlugin::group() const
    return tr("Image Manipulation Widgets");

The group() function returns the name of the widget box group to which this custom widget should belong. If the name isn't already in use, Qt Designer will create a new group for the widget.

QIcon IconEditorPlugin::icon() const
    return QIcon(":/images/iconeditor.png");

The icon() function returns the icon to use to represent the custom widget in Qt Designer's widget box. Here, we assume that the IconEditorPlugin has an associated Qt resource file with a suitable entry for the icon editor image.

QString IconEditorPlugin::toolTip() const
    return tr("An icon editor widget");

The toolTip() function returns the tooltip to show when the mouse hovers over the custom widget in Qt Designer's widget box.

QString IconEditorPlugin::whatsThis() const
    return tr("This widget is presented in Chapter 5 of <i>C++ GUI "
              "Programming with Qt 4</i> as an example of a custom Qt "

The whatsThis() function returns the "What's This?" text for Qt Designer to display.

bool IconEditorPlugin::isContainer() const
    return false;

The isContainer() function returns true if the widget can contain other widgets; otherwise, it returns false. For example, QFrame is a widget that can contain other widgets. In general, any Qt widget can contain other widgets, but Qt Designer disallows this when isContainer() returns false.

QWidget *IconEditorPlugin::createWidget(QWidget *parent)
    return new IconEditor(parent);

Qt Designer calls the createWidget() function to create an instance of a widget class with the given parent.

Q_EXPORT_PLUGIN2(iconeditorplugin, IconEditorPlugin)

At the end of the source file that implements the plugin class, we must use the Q_EXPORT_PLUGIN2() macro to make the plugin available to Qt Designer. The first argument is the name we want to give the plugin; the second argument is the name of the class that implements it.

The .pro file for building the plugin looks like this:

TEMPLATE     = lib
CONFIG      += designer plugin release
HEADERS      = ../iconeditor/iconeditor.h                iconeditorplugin.h
SOURCES      = ../iconeditor/iconeditor.cpp                iconeditorplugin.cpp
RESOURCES    = iconeditorplugin.qrc
DESTDIR      = $$[QT_INSTALL_PLUGINS]/designer

The qmake build tool has some predefined variables built into it. One of them is $$[QT_INSTALL_PLUGINS], which holds the path to the plugins directory inside the directory where Qt is installed. When you type make or nmake to build the plugin, it will automatically install itself in Qt's plugins/designer directory. Once the plugin is built, the IconEditor widget can be used in Qt Designer in the same way as any of Qt's built-in widgets.

If you want to integrate several custom widgets with Qt Designer, you can either create one plugin for each of them or combine them into a single plugin by deriving from QDesignerCustomWidgetCollectionInterface.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020