And yet the example give with Qt, stays with Qt only. I can't reuse any code written with Qt classes, unless I want to use Qt. But I can reuse small libraries

And yet the example give with Qt, stays with Qt only. I can't reuse any code written with Qt classes, unless I want to use Qt. But I can reuse small libraries almost anywhere - zlib, libtommath, tomcrypt, lzo1x, etc. The smaller they are, often it's easier, and the less platform specific they use, or alternatively the less platform-specific primitives they use the more reuse. Reuse in binary sense, or blackbox-ish sense. Not really extending them, just putting them to use as they are.