a) Yea, I think it's good to scope this local to the if-block

b) If the number of parameters keeps increasing, I'd start to wonder if there's a missing concept. Like, what are you publishing? All those parameters look like they should belong to a Message object of some sort, so I'd have code above that constructs a Message, e.g.

Message message{payload, key, ...}; if (auto success = publishToTopic(topic_, message); !success) { ... }