Qt philosophy to using or not using 'const'

  • I stumbled upon a specific example of "why didn't they make that const?":

    I create a QTextLayout for a custom paint engine (in my case, a printer)

    QTextLayout layout(text, font, m_Printer);

    m_Printer is a class member. However, I'd like to use a text layout in some pre-calculation method, and didn't see why I shouldn't make that method const:

    qreal MyClass::calculateTextBlockHeight(const QString& text, const QFont& font) const
       QTextLayout layout(text, font, m_Printer);
       // Some layout code
       return calculatedHeight;

    What stops me from making the method const: QTextLayout expects a non-const QPaintDevice as 3rd parameter. I wonder: What would QTextLayout want to change on my paint device?

    Turns out it doesn't want to change anything: The paint device is just used in a QFont-constructor internally (which also takes it non-const), and then all the code does is read the logical DPI. So both constructors of QFont and QTextLayout would be perfectly happy with a const QPaintDevice*, and so would I.

    Would you consider this an oversight, or is there some coding standard or interface philosophy behind?

  • It's a good suggestion for a change in Qt 6, add it in https://bugreports.qt.io

    In the meantime, given, as you noted correctly, the paint device is used only as it was const it's still safe to const-cast it and pass it: QTextLayout layout(text, font, const_cast<MyClass*>(this)->m_Printer);

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