Pretty-Printing Output for QCOMPARE

We have created a custom class, say Person, and use it in a unit test.

void TestPerson::testEquality()
    Person p1("Alice", 42);
    Person p2("Bob", 37);
    QCOMPARE(p1, p2);

The unit test fails with this message.

FAIL!  : TestPerson::testEquality() Compared values are not the same
   Loc: [../QComparePrint/TestPerson.cpp(8)]

This message does not tell us how the two Person objects differ. We would like to see this message as we would see it for any type known to QCOMPARE.

FAIL!  : TestPerson::testEquality() Compared values are not the same
   Actual   (p1): "Person(Alice, 42)"
   Expected (p2): "Person(Bob, 37)"
   Loc: [../QComparePrint/TestPerson.cpp(16)]

How do we achieve such a pretty-printed output for QCOMPARE?
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Passing Enum Properties between C++ and QML

We have defined a Qt property warningLevel in the C++ class MainModel:

    Q_PROPERTY(WarningLevel::Enum warningLevel READ warningLevel
               WRITE setWarningLevel NOTIFY warningLevelChanged)

We want to use this property in QML. For example, we want to colour a rectangle according to the warningLevel:

    import com.embeddeduse.models 1.0
    // ...

    property MainModel mainModel : MainModel {}

    Rectangle {
        color: toColor(mainModel.warningLevel)
        // ...

    function toColor(level) {
        switch (level) {
        case WarningLevel.Error:
            return "red"
        case WarningLevel.Warning:
            return "orange"
        case WarningLevel.Info:
            return "green"
        case WarningLevel.Debug:
            return "purple"
            return "magenta"

Note how we access the C++ property mainModel.warningLevel from QML to set the color of the rectangle and how we use symbolic enum constants like WarningLevel.Info in the function toColor().

It is similarly easy to use a list of the symbolic enum constants as the model of a Repeater and to assign the warning level by the user to the property mainModel.warningLevel in the onReleased handler of a MouseArea.

    Repeater {
        model: [WarningLevel.Error, WarningLevel.Warning, WarningLevel.Info,
        Rectangle {
            color: toColor(modelData)
            // ...
            MouseArea {
                anchors.fill: parent
                onReleased: mainModel.warningLevel = modelData

I’ll show you in the rest of this post how to write your C++ code so that you can use a C++ property of enum type easily in QML.
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Announcing My Talk “Qt vs. Web – Total Cost of Ownership” at Qt World Summit 2017

I am proud to announce that my talk “Qt vs. Web – Total Cost of Ownership” was accepted for Qt World Summit 2017. It is scheduled for Thursday, October 12, at 11:30 in the Business track.

I’ll show that Web applications require a considerably more powerful system-on-chip (SoC) than Qt applications to achieve a good user experience. However, a more powerful SoC is also much more expensive, as the following diagram shows.

Hope to see you in Berlin.

I’ll make the presentation slides available immediately after Qt World Summit.

More Reasons For Not Using Web

The Qt Company published a guest post “Qt QML v HTML5 – a practical comparison” and a whitepaper by the Austrian Qt consultancy Sequality. Sequality had one developer writing a simplified application for controlling a bottling plant first with Qt and then with Web (AngularJS). The developer had 160 hours for each implementation. The application had to run on a tablet, a PC and a Raspberry Pi 3 with different resolutions.

Here are my favourite findings of Sequality’s experiment.

  • The developer finished considerably more functionality with QML than with Web.
  • Higher efforts to use OpenGL acceleration for Web: “Enabling GPU rendering on Chromium […] doesn’t fix the HTML5 demo’s performance problem. In fact, the CPU is utilized even more, which leads to overheating.”
  • Higher efforts for testing Web: “The fact that HTML5 applications can be executed on a number of platforms – and a number of browser engines on each platform – multiplies the testing time correspondingly.”
  • Availability of a certain Web technology in 10 years: “Modern HTML5-based applications that use frameworks like AngularJS are relatively new and undergo changes from year to year – a valid question is whether AngularJS (or any other currently trendy Javascript-library) will still be a relevant HTML5-technology in 10 years.”

Qt vs. Web (Part 1): Three Cautionary Stories About Web

Let XXX be a manufacturer of printers, TVs, set-top boxes (STBs) or home appliances. XXX must decide whether to use Qt or Web for the HMI of their devices. I’ll start my series “Qt vs. Web” with three cautionary stories about using Web technologies. Facebook moved from Web to native. Netflix built its own rendering engine and JavaScript HMI library to continue with Web technologies. Although LG’s smart TVs run on WebOS, the built-in and brand-critical TV UI uses QML and Qt. Third-party apps use standard Web technologies.
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Boot2Qt Open-Sourced as Part of Yocto

Tuukka Turunen from The Qt Company shared some fantastic news in his post Aligning with the Yocto Project.

We have created a separate Boot to Qt meta layer, meta-boot2qt, which takes care of building the images and toolchains for the reference devices. The meta-boot2qt layer integrates all the required BSP meta layers, so there is no manual configurations necessary when starting Yocto build for one of the Qt reference devices. The layer was previously available only for our commercial customers, but with Qt 5.7 we have open sourced it as well.

The Boot2Qt meta layer comes with support for all three versions of the Raspberry Pi, many i.MX6 boards (Nitrogen6x, SABRE, Colibri, etc.), the NVIDIA Drive CX and for the Intel NUC. This should make the live of embedded Qt developers a lot easier. Thank you very much, Trolls!

Best Friends: C++11 Move Semantics and Pimpl

Move semantics is faster than copy semantics, when the compiler can replace expensive copy operations by cheaper move operations, that is, when it can replace a deep copy of a big object by a shallow copy of the pointer to the big object. Hence, classes using the pimpl idiom in combination with move semantics should see a considerable speed-up. As Qt applies the pimpl idiom consistently to every non-trivial Qt class, we should see a speed-up by simply using Qt classes instead of their STL counterparts. I’ll compare the performance of classes that use move semantics with Qt and STL classes with and without applying the pimpl idiom.
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Simplifying Loops with C++11 in Qt Ways

Recently, I looked through the code base of a medium-sized project to see how I could simplify handwritten for-loops by using C++11’s new range-based for and STL algorithms with lambda expressions. The results in short: Range-based for makes loops simpler, easier to understand and often faster. STL algorithms are often a bit harder to read and write than range-based for loops, because lambda expressions are pretty clumsy, algorithm naming is inconsistent, and algorithm interfaces are inconvenient. But, they are still better than handwritten for-loops.
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