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What exactly is QIcon::cacheKey based on?

  • I have a cache of icons in my application. Naturally, I use QIcon::cacheKey. And I'm starting to suspect it doesn't do what I think it should. There are like 20-50 different icons, and yet my map<QIcon::cacheKey, QIcon> has 3000 entries. It's as if the hash (cacheKey) is based on the Pixmap object (which I've got a new one for every icon), rather than the actual image data.
    Consequently, what is the fastest way to obtain an image-data-based hash for caching QIcons? This part of the application is already underperforming, so hashing must be fast.

  • Lifetime Qt Champion


    It's not based on either, You should take a look at QIcon's implementation for the current algorithm used.

    How are you adding these icons to your cache ?

  • Yep, it definitely doesn't do what I thought it would.
    I'm requesting the icons using QFileIconProvider, and simply add them to the map:

    QIcon icon = _provider.icon(filePath);
    _cahce[icon.cacheKey()] = icon;

    If 2 different files have the same icon, which is very common, the icons I get will have the same image, but still different key - because it's not based on the contents.

  • This works, but I wonder if it's the fastest way.

    QCryptographicHash qCryptoHash(QCryptographicHash::Md5);
    const auto qimage = icon.pixmap(icon.availableSizes().front()).toImage();
    qCryptoHash.addData((const char*)qimage.constBits(), qimage.bytesPerLine() * qimage.height());
    const auto result = qCryptoHash.result();
    const qulonglong iconHash = *(qulonglong*)( ^ *(qulonglong*)(;

  • Lifetime Qt Champion

    No it's not, but since it should only be done once that might be enough

  • No, it's not done once. I take hash of a QIcon every time I need to look it up in the cache. Basically, I'm calculating hashes all the time.
    How can I optimize the code?

  • Lifetime Qt Champion

    Why not keep it with the icon after calculating at construction time ?

    How are you using that cache ? It sounds strange that you get 3000 different entries.

  • I no longer have 3000 entries after introducing the hash function that accounts for the image content.
    I can't calculate the hash any less often because I'm getting new and new icons from QFileIconProvider, and I need to hash their contents to find out if I already have this icon in the cache.

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