Implicit sharing and mutable members



  • How do you handle mutable attributes in a class that implements implicit sharing?

    A case in point: I have a class that represents a text entry. It allows properties to be set that define how the text looks, and offers a paint method to paint the text using a painter provided as a parameter, and a QRectF to paint into.

    I use this class a lot, so I implemented implicit sharing. My private data object looks like this:

    @class CGLib_TextEntryPrivate : public QSharedData
    {
    public:
    // Constructors, Destructor omitted in this example
    // Just assume they are here and correct.

    QString m_Text;
    QFont m_Font;
    QColor m_Color;
    Qt::Alignment m_Alignment;
    qreal m_Width;
    QTextOption::WrapMode m_WordWrapMode;
    
    mutable QStaticText m_StaticText;
    // m_StaticText.size() is unreliable
    // therefore I do my own size calculation
    mutable QSizeF m_Size;
    mutable bool m_bStaticTextValid;
    mutable QTransform m_LastUsedTransform;
    

    };@

    The members m_StaticText and following are basically only caching information. I do not want to update them every time a property is changed, but as late as possible, as rarely as possible.

    This can lead to situations where I have to update them in a getter. I have the choice of either making the getter non-const (which leads to lots of problems when using the class in a const-correct code environment), or making those "caching" members mutable.
    I chose the latter. Can I run into implicit sharing troubles that way?

    The main class has, of course, a shared data pointer:
    @QSharedDataPointer<CGLib_TextEntryPrivate> d;@

    Whenever I change anything that must lead to a cache update, I call an invalidation function, which is non-const:
    @void CGLib_TextEntry::invalidateStaticText()
    {
    d->m_bStaticTextValid = false;
    }@

    In some of the getters, I call an update function which changes only mutable members, and is const:
    @QSizeF CGLib_TextEntry::size() const
    {
    updateStaticText(); // This is a const function I am calling

    return d->m_Size;
    

    }@

    I might just as well be fine with the way the class is right now: The caching information can only change after one of the non-mutable members has changed. This change must have already caused the detach to happen, which means I am updating the cache information in an object which is already detached. The question is: Are there any holes in this reasoning?



  • I don't see any holes, currently. Only the one with detach. changing members implicitly, which might affect other users of the shared object looks not the best.

    But it could work.


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