ibqtxdg is a Qt 5 implementation of freedesktop.org XDG specifications.

It is maintained by the LXQt project and nearly all LXQt components are depending on it. Yet it can be used independently from this desktop environment, too.

The library is able to use GTK+ icon theme caches for faster icon lookup. 
The cache file can be generated with utility gtk-update-icon-cache on a theme directory. 
If the cache is not present, corrupted, or outdated, the normal slow lookup is still run.