[PATCH] MarbleDebug: don't use a class extending QIODevice as a null device

[Kai Koehne]

> -----Original Message-----
> From: Thiago Macieira [mailto:thiago at kde.org]
> Sent: Tuesday, April 18, 2017 6:36 PM
> To: Stefan Fuchs <sfuchs at gmx.de>
> Cc: Lubomir I. Ivanov <neolit123 at gmail.com>; Subsurface Mailing List
> <subsurface at subsurface-divelog.org>; Kai Koehne <Kai.Koehne at qt.io>
> Subject: Re: [PATCH] MarbleDebug: don't use a class extending QIODevice as
> a null device
> 
> Em terça-feira, 18 de abril de 2017, às 09:23:12 PDT, Stefan Fuchs escreveu:
> > /home/stefan/Entwicklung/Subsurface/marble-
> source/src/lib/marble/Marbl
> > eDebug
> > .cpp:27:53: error: passing 'const QLoggingCategory' as 'this' argument
> > of 'void QLoggingCategory::setEnabled(QtMsgType, bool)' discards
> > qualifiers [-fpermissive] loggingCategory().setEnabled(QtDebugMsg,
> > enabled);
> 
> Hold on. This one is weird, because how can you call that non-const method
> if everything is const?
> 
> Kai, how is one supposed to call QLoggingCategory::setEnabled()? Or are we
> not supposed to?
>
> Or should we skip the Q_LOGGING_CATEGORY macro and deploy our own
> code?

The general idea is that the logging category objects represent a view on the general logging configuration, and shouldn't be manipulated directly. That is, if you call setEnabled() on one object it will be only changing the exact object, and not another object that might represent the same category. Also, a change in the logging configuration might overwrite your local change at any time.

To avoid this, I suggest to change the logging rules either through custom logging rules (QLoggingCategory::setFilterRules(), QT_LOGGING_RULES, QT_LOGGING_CONF), or by installing a custom logging filter (QLoggingCategory::installFilter()). If you 'just' want to set a default logging level (so that e.g. QtDebugMsg messages are not printed for your categories) you can also set a level in your Q_LOGGING_CATEGORY macro call.

This is also all documented in http://doc.qt.io/qt-5/qloggingcategory.html#details . Please raise it if you feel that anything is unclear/missing there.

Regards

Kai