Depends on the definition of an addict. If you mean someone who shoots dope 24/7, then no I haven't seen functional ones. If it's just someone who is compelled to use (almost) every day, then yes. My father and older brother both are functional alcoholics. They, especially my dad, drink every evening, but it has never interfered with their work. I don't even think many people know they drink outside of their family circles.
I myself am not even sure if I fit the criteria for being an addict. I crave drugs every day, but then again I don't mind if I miss a day or two if it so happens - I'm not very physically dependent because of my reasonably low doses, so I don't get nasty withdrawals. Lately though I can't remember a period longer than 3-4 days when I went without getting high. I drink beer or do opioids, lately it has only been opioids. Finally been able to quit drinking. Since I completely switched to opis my drug use has stopped impacting my professional and social life as it did on alcohol. Personal life though? Uhh, opioids make me emotionally quite numb so I've done things I probably wouldn't have otherwise done... But hey who needs relationships when you have opioids.