Crash with QFontMetrics::elidedText

[ajaxcrypto]

Calling QFontMetrics::elidedText leads to a crash (call on GUI thread), which crash coming from QFontPrivateData.
Here is the callstack:

 Qt5Guid.dll!QAtomicOpsBySize<4>::ref<int>(int & _q_value) Line 308	C++
 Qt5Guid.dll!QBasicAtomicInteger<int>::ref() Line 113	C++ 
 Qt5Guid.dll!QExplicitlySharedDataPointer<QFontPrivate>::QExplicitlySharedDataPointer<QFontPrivate>(QFontPrivate * adata) Line 264	C++
 Qt5Guid.dll!QFont::QFont(QFontPrivate * data) Line 565	C++
 Qt5Guid.dll!QFontMetrics::elidedText(const QString & text, Qt::TextElideMode mode, int width, int flags) Line 905	C++

This is on Windows 10, Qt 5.9.1 with MSVC 2015 (No optimization flags enabled)

[Christian Ehrlicher]

Please provide a minimal, compilable example.

[SGaist]

Hi,

Please provide a minimal compilable example that triggers the crash.

You should also consider updating your version of Qt. Qt 5.9.1 is quite old.

[ajaxcrypto]

static const QFont font{ "Consolas", 9, 100 };

// This function is frequently called, font and width remain constant
std::pair<int, const QFontMetrics&> GetInfo() 
{
    static QFontMetrics metrics{font};
    static int width = metrics.averageCharWidth() * MAX_CHARS_REQUIRED; // some constant
    return std::make_pair(width, metrics);
}

int main(int argc, char** argv)
{
    QApplication app(argc, argv);
    auto pair = GetInfo();
    auto txt = pair.second.elidedText("some long text", Qt::ElideRight, pair.first); // crash here
    std::cout << text.toStdString();
    return app.exec();
}

[ajaxcrypto]

@SGaist Unfortunately, I can't update the Qt version. We might eventually mode to Qt6 (at a later date)

[SGaist]

That code has an issue, it's missing a QApplication object. It's mandatory to setup Qt's internals that needs to access graphic system related data/information.

[ajaxcrypto]

@SGaist I ommitted certain pieces. QApplication as well as QWindow exists in the real one.

[SGaist]

Here is the issue: with missing bits and pieces, your minimal application does not reflect your situation so it is likely hiding the real issue.

[ajaxcrypto]

I can't really copy paste the entire application code here. But what are the possible scenarios in which I can face error from QFontPrivate ? Or it too vague to guess?

[Christian Ehrlicher]

You return a dangling QFontMetrics reference.

[Christian Ehrlicher]

To be more precise: you're failing to due std::make_pair: "The deduced types V1 and V2 are std::decay<T1>::type and std::decay<T2>::type"

So either use return std::pair<int, const QFontMetrics &>(width, metrics); or since you prefer (although very very inconsistent) brace initialization return {width, metrics};