Friday, July 17, 2020

Serialize/Deserialize JSON Representations to Qt Objects (QObject's) in C++

Calls to HTTP servers are very frequent nowadays, and JSON is frequently used when dealing with structured data. When I write applications in Java/Kotlin, I always use retrofit2 with gson integration, or similar approaches. It is very comfortable to be able to serialize/deserialize to Java objects automatically. I tried to find something similar for Qt, but I couldn't find much. JSON support in Qt is good, but does not seem to be able to map QOject's to QJSonObject. I also could not find other C++ libraries doing this. Probably the difficulty of implementing reflection in C++ would make the implementation a bit convoluted. A good candidate I found is this:, which seems not to require much boilerplate code, not too verbose and no generated code.

However, Qt includes the meta object system (, and therefore I tried to use it to get the desired result. In a couple of hours I got to a pretty decent result: It is far from perfect, but it seems to work. Have a look at the readme or the unit tests to know more.
The draft is free to use: you can contribute, report bugs etc...

For instance, a JSON object like this:

{"menu": {
    "header": "SVG Viewer",
    "items": [
        {"id": "Open"},
        {"id": "OpenNew", "label": "Open New"},
        {"id": "ZoomIn", "label": "Zoom In"},
        {"id": "ZoomOut", "label": "Zoom Out"},
        {"id": "OriginalView", "label": "Original View"},
        {"id": "Quality"},
        {"id": "Pause"},
        {"id": "Mute"},
        {"id": "Find", "label": "Find..."},
        {"id": "FindAgain", "label": "Find Again"},
        {"id": "Copy"},
        {"id": "CopyAgain", "label": "Copy Again"},
        {"id": "CopySVG", "label": "Copy SVG"},
        {"id": "ViewSVG", "label": "View SVG"},
        {"id": "ViewSource", "label": "View Source"},
        {"id": "SaveAs", "label": "Save As"},
        {"id": "Help"},
        {"id": "About", "label": "About Adobe CVG Viewer..."}

can be deserialized to a QObject simply by defining the classes (macros here are inherited by my other project, but this is not mandatory):

L_RW_PROP(QString, id, setId, QString())
L_RW_PROP(QString, label, setLabel, QString())

L_RW_PROP(QString, header, setHeader)
L_RW_PROP_ARRAY_WITH_ADDER(Item*, items, setItems)

L_RW_PROP(Menu*, menu, setMenu, nullptr)

and by writing these few lines:

LDeserializer<MenuRoot> deserializer(factory);
QScopedPointer<MenuRoot> g(deserializer.deserialize(jsonString));

Please leave a comment if you know of other tools in this context, serializing and deserializing JSON is very frequent.
Bye! ;-)

Sunday, June 14, 2020

Synthesize Qt Settings

I frequently create classes that I use as an interface to settings in my apps. Using a single class with accessors makes the code readable, safer and simple to maintain. Unfortunately, doing this requires to create getters and setters for every entry in the settings file, which is a bit bothering. Also, QSettings is not a QObject and cannot provide signals to notify changes.

My use cases are typically very simple, so I created a couple of macros that synthesise classes for me. Macros synthesise a reentrant class that can be used to access settings and a notifier, that can be used to get notifications of the changes. An example of definition of the class is:

L_DECLARE_SETTINGS(LSettingsTest, new QSettings("settings.ini", QSettings::IniFormat))
L_DEFINE_VALUE(QString, string1, QString("string1"), toString)
L_DEFINE_VALUE(QSize, size, QSize(100, 100), toSize)
L_DEFINE_VALUE(double, temperature, -1, toDouble)
L_DEFINE_VALUE(QByteArray, image, QByteArray(), toByteArray)

L_DECLARE_SETTINGS(LSettingsTestSec1, new QSettings("settings.ini", QSettings::IniFormat), "SECTION_1")
L_DEFINE_VALUE(QString, string2, QString("string2"), toString)

this will let you instantiate objects of class LSettingsTest and LSettingsTestSec1, and access entries with strong typed methods. Also, by calling LSettingsTest::notifier(), you can get a reference to the unique notifier. By setting an instance as a context property, you can get notifications and you can update settings from QML. You can find some more info in the repo and an example using Qt Quick here:
Bye! ;-)

Thursday, May 28, 2020

Synthesize Qt properties

The are some utils that I use in most of my Qt projects. I'm a bit fed up of copy-pasting those every time I need something, so I created a project containing tools that are of frequent use when I write Qt apps, regardless of the type of app.

The only interesting tool I added yet is the synthesizer of Q_PROPERTY's. You can use the macros:
to synth├ętise props, with getter, setter and notifications. Each macro is overloaded: with 3 params, you do not have initialisation, adding a 4th param, also initialises the variable to the provided value. Also, I frequently need QObject that are just containers of properties, to be used in QML. I added these two macros to spare some lines:
A class like:
class Fraction : public QObject
    Q_PROPERTY(double numerator READ numerator WRITE setNumerator NOTIFY numeratorChanged)
    Q_PROPERTY(double denominator READ denominator WRITE setDenominator NOTIFY denominatorChanged)
    Fraction(QObject* parent = nullptr) : QObject(parent) {}
    double numerator() const {
        return m_numerator;
    double denominator() const {
        return m_denominator;
public slots:
    void setNumerator(double numerator) {
        if (m_numerator == numerator)
        m_numerator = numerator;
        emit numeratorChanged(numerator);
    void setDenominator(double denominator) {
        if (m_denominator == denominator)
        m_denominator = denominator;
        emit denominatorChanged(denominator);
    void numeratorChanged(double numerator);
    void denominatorChanged(double denominator);
    double m_numerator;
    double m_denominator;
can be simplified to:
L_RW_PROP(double, numerator, setNumerator)
L_RW_PROP(double, denominator, setDenominator)
I'm sure there are other similar approaches out there, but if you need it, you can simply add this repo as a submodule like I do:
Bye! ;-)

Monday, September 9, 2019

Raspbian Buster and Cross Toolchain for Linux and Mac OS 8.3.0

Raspbian Buster is out. It seems it updated gcc from version 6.3.0 (used for Stretch) to gcc version 8.3.0. This is not always needed, but I had a lot of problems in the past related to using an outdated crosscompiler, or the one provided in the raspberry repo: I therefore built my own toolchain for Stretch, which I provided here, both for Linux and Mac OS.

I did the same for Buster: I built my own toolchain and I built many projects with it, like Qt, ffmpeg etc... Just place the toolchain in /opt/rpi (probably position independent) and you should be done. This toolchain should be 100% compatible with gcc provided by Buster.

Download cross toolchain GCC 6.3.0 for Stretch here.
Download Linux x64 cross toolchain GCC 8.3.0 here.
Download Mac OS cross toolchain GCC 8.3.0 here.

Bye ;-)

Sunday, March 4, 2018

Yocto Recipe for Accelerated Video Playback with Qt on Raspberry Pi

Raspberry has a huge community that creates a considerable amount of material in a large number of projects. A relevant support for Yocto cannot therefore be missing for it. Months ago I started to work on using Yocto to create images for the Raspberry Pi. Unfortunately I had to stop for a few months to work on other projects, so the result could not be published immediately: the project is based on Pyro while current Yocto release is Rocko. The concept is however similar.

I created a test image for the Raspberry Pi 3 board, using the best compilation flags I found available, and added a recipe for POT to include hardware acceleration in Qt. The result is good: just invoke qmlscene on a little QML code and you have a hardware accelerated OpenGL scene rendering video with Qt API (QML) in your Yocto image.

You can find the layer here: It is based on the POT project that can be found here:


In the repo I also added a couple of examples of images I created for me. I'll improve those in case I'll find the time to do it.

All you have to do is include the recipe and POT will be installed in the image (satisfy the dependencies if some were missing).


I created some images using this setup (I included many things that are not mandatory at all for POT):

   ${WORKSPACE}/poky-pyro/meta \

   ${WORKSPACE}/poky-pyro/meta-poky \
   ${WORKSPACE}/poky-pyro/meta-openembedded/meta-oe \
   ${WORKSPACE}/poky-pyro/meta-openembedded/meta-multimedia \
   ${WORKSPACE}/poky-pyro/meta-openembedded/meta-networking \
   ${WORKSPACE}/poky-pyro/meta-openembedded/meta-python \
   ${WORKSPACE}/poky-pyro/meta-raspberrypi-crypto \
   ${WORKSPACE}/poky-pyro/meta-qt5 \
   ${WORKSPACE}/poky-pyro/meta-oracle-java \
   ${WORKSPACE}/poky-pyro/meta-java \
   ${WORKSPACE}/poky-pyro/meta-office \
   ${WORKSPACE}/meta-pot \

in my layer.conf I placed:

LICENSE_FLAGS_WHITELIST = "commercial oracle_java"
DISTRO_FEATURES = "ext2 ext3 ext4 pam gles2 usbhost ${DISTRO_FEATURES_LIBC}"
PACKAGE_CLASSES = "package_deb"
IMAGE_FEATURES += "package-management ssh-server-openssh"
DISTRO_FEATURES_remove = "X11 wayland"
EXTRA_IMAGE_FEATURES = "debug-tweaks dev-pkgs tools-sdk tools-debug"
PREFERRED_PROVIDER_jpeg = "libjpeg-turbo"
PREFERRED_PROVIDER_jpeg-native = "libjpeg-turbo-native"
PREFERRED_PROVIDER_virtual/java-initial-native = "cacao-initial-native"
PREFERRED_PROVIDER_virtual/java-native = "cacao-native"
PREFERRED_PROVIDER_virtual/javac-native = "ecj-bootstrap-native"
VIRTUAL_RUNTIME_init_manager = "sysvinit"
MACHINE_FEATURES_remove = "apm"
IMAGE_FSTYPES ?= "rpi-sdimg"
MACHINE = "raspberrypi3"
DISTRO = "poky"

At this point you can use something like pot-minimal ( and you should have a working image.

You can test it pretty simply by using a trivial QML like this:

import QtQuick 2.2
import QtMultimedia 5.5

Rectangle {
   color: "orange"

   Video {
      autoLoad: true
      autoPlay: true
      source: "file:///home/root/"
      anchors.fill: parent

and you should see your video in the QML scene right away with the command:

qmlscene -platform eglfs main.qml


I wanted to also create an image fully optimized for the Raspberry Pi 3 board, which is a Cortex-A53 armv8-a architecture which includes crc and new A64, A32, and T32 instructions to Advanced SIMD that accelerate Advanced Encryption Standard (AES) encryption and decryption, and the Secure Hash Algorithm (SHA) functions SHA-1, SHA-224, and SHA-256. Still I wanted a fully 32-bit system as hardware libraries are only supported for this bitness. To do this I had to apply a few patches to the Yocto repos. I uploaded every patch to these forks: (branch: pyro_cortexa53) (branch: pyro_cortexa53)

Bye! ;-)

Tuesday, January 2, 2018

POT 5.7.0 with Qt 5.10.0 built for armv8 with GCC 6.3.0 on Raspbian Stretch

This was a good test for my cross toolchain with gcc 6.3.0 I uploaded some days ago: the new Qt 5.10.0 built with -march=armv8-a -mtune=cortex-a53 -mfpu=crypto-neon-fp-armv8 -mfloat-abi=hard, including POT for video acceleration on Raspbian Stretch. The build includes most Qt modules plus Qt WebKit (the modernized QtWebKit from
Konstantin Tokarev, version 5.212, plus QtWebEngine 5.9.1, a bit patched to build here. I didn't test browsers much, you'll have to work on those yourself if you need them.

By using the Qt build and the cross toolchain you can build your own applications.

Download here the build for Qt 5.10.0 with POT 5.7.
Download here the cross toolchain.

Friday, November 24, 2017

Raspbian Stretch and Cross Toolchain for Linux and Mac OS

Raspbian Stretch is out for Raspberry Pi. I needed a cross toolchain with the same exact version used to build the system so I built my own. As I had my script ready, I also tried to build one for Mac OS and it seems to work for the moment. I tested these cross toolchains for my projects and seemed to be able to build everything that I tried: Qt libs, apps, omxplayer, ffmpeg etc...

Download Linux x64 cross toolchain GCC 6.3.0 for Stretch here.
Download Mac OS cross toolchain GCC 6.3.0 for Stretch here.

NOTE: The Mac OS version requires some packages to be installed from macports.
NOTE: The toolchain is built to crossbuild for armv7-a. I could also build for armv8 but not sure if it also works for armv6.

Have fun! Bye! ;-)