I am writing an application and there will potentially be tens of thousands of labels (a log-viewing application of a sort), most of them hidden with QWidget::hide(). I imagine a QLabel, when created, takes up some video memory. Now, does hide() free that video memory? Or will I have to QWidget::remove() most of those hidden labels to keep video memory usage at a reasonable level?
In general, most widgets do not store their pre-rendered images in memory. Instead, they are render themselves on demand after being invalidated. However, some do it if render is time-consuming. Took a look at QLabel source code (http://code.qt.io/cgit/qt/qtbase.git/tree/src/widgets/widgets/qlabel.cpp), it seems that QLabel caches its pixmap when scaledContents is enabled and scaling is necessary. Plain text-only labels are painted as-is without any caching.
Still, as @G.M mentioned, each widget consumes some system memory to store its own data, and processing time due to event handling, so producing 10k labels is a reasonable resource waste. In contrast, item views are single widgets that draws items on their surface. No event handling overhead, no unnecessary caches. Like QLabels, item view items are perfectly stylable, see http://doc.qt.io/archives/qt-5.8/stylesheet-examples.html#customizing-qlistview, http://doc.qt.io/archives/qt-5.8/stylesheet-examples.html#customizing-qtreeview for details. More complex looks like multi-line list items are achievable with QItemDelegate: Qt QListWidgetItem Multiple Lines