When you create your own KDE application you will probably want to have icons in it respecting your user's icon theme.
Plasma and all the other major Linux deskops follow the Freedesktop icon specification and naming specification. A fallback mechanism is also present so that if the
document-open-recent icon is not present, the
document-open icon will be displayed instead, and if the
document-open icon is also not present the
document icon will be displayed.
Finding the right icons
KDE provides an application to search for icons, Cuttlefish. It is generally available in the
plasma-sdk package available through your package manager on Linux.
Cuttlefish allows you to search for icons by name and by category. You can inspect the different color schemes of an icon as well as view a selected icon in other themes.
In a traditional QtWidgets application:
QIcon undoicon = QIcon::fromTheme("edit-undo");
In a QtQuick application:
import org.kde.kirigami 2.14 as Kirigami
Other platforms than Linux
On other plateforms, the icon theme is not available.
On Android, you can use the kirigami_package_breeze_icons() CMake function to package the Breeze icons you are using.