Using QSettings with data stored in a QByteArray, or QIODevice

[Guy Gizmo]

I have data in ini format stored in a QByteArray, and I'd like to be able to read and write to it using QSettings. Unfortunately, out of the box QSettings only seems capable of working with files specifically, and can't be used with a QByteArray or an instance of a class that derives from QIODevice such as QBuffer.

So far, I've found two ways one way of dealing with this:

1. Dump the data temporarily into a file, probably using QTemporaryFile, and read that using QSettings
2. Create a custom implementation of QAbstractFileEngine that can read from the data in the byte array as though it were a file, as suggested in this ten year old thread. (edit: I've just discovered that QAbstractFileEngine is no longer public in Qt 5, so it's not a viable solution for me.)

Neither of these options are very appealing. Ten years has gone by since the suggestion of using QAbstractFileEngine. Is there a better method available now?

If this is currently not possible, consider this a feature suggestion that QSettings be able to work with anything derived from QIODevice that has the necessary capabilities.

[Christian Ehrlicher]

Use a QTemporaryFile and provide a patch to allow a QIODevice.

[Guy Gizmo]

Writing a patch would be a massive undertaking and one I most definitely don't have time for right now. 😝

I'm still curious if there's a method available other than using QTemporaryFile.

[Bonnie]

@Guy-Gizmo said in Using QSettings with data stored in a QByteArray, or QIODevice:

consider this a feature suggestion

This is an user forum.
If you want to give suggestion to dev team you should file an issue to https://bugreports.qt.io

[fcarney]

@Christian-Ehrlicher said in Using QSettings with data stored in a QByteArray, or QIODevice:

provide a patch

Are you talking about making a format and registering it?

I was thinking something from boost would be helpful. Not sure how the format interface works though.

[Guy Gizmo]

@Bonnie I just checked my account on bugreports.qt.io and found I made just such a request back in February 2016. Hasn't gotten any votes since then. I doubt this feature will be added, sadly!

[Guy Gizmo]

@fcarney said in Using QSettings with data stored in a QByteArray, or QIODevice:

I was thinking something from boost would be helpful. Not sure how the format interface works though.

I considered adding another format that uses a separate ini parser that can handle data in memory, but that only gets me halfway there. I need to be able to read ini formatted files already written using QSettings, which means that certain types (for example QByteArray, QStringList, etc.) have been encoded using a method specific to QSettings' ini formatter. As far as I know QSettings doesn't expose its facility for encoding and decoding those values.

[fcarney]

@fcarney said in Using QSettings with data stored in a QByteArray, or QIODevice:

something from boost

That is why I linked a cross platform INI file reader/writer from boost "something from boost".

[Guy Gizmo]

Just a quick follow-up: I ended up using QAbstractFileEngine to solve this anyway, even though it's private in Qt 5.15. If it ever is removed or otherwise stops becoming a viable method, I'll update my code accordingly.

That said, if anyone can suggest another viable method that doesn't rely on writing the data out to a temporary file, I'd love to hear it!