I'd like to +1 the advocates of -x c++ here. There's just no reason not to. It's like building scaffolding around a building so you can retrofit it. It doesn't need to end up like a tower of c++, but the scaffolding lets you hoist things out of overly complex codebases a lot easier. The only immediate code changes encountered were adding explicit casts from malloc (and fixing actual bugs that had been hidden.) In one case recently, trivially converting some c structures to c++ and hiding internal representations let me delete thousands of lines of set(foo, xxx) calls because I could prove that there was no access to the computed result.

However, because I'm inherently chaotic neutral, I tend to use -x objective-c++, but that's a story for another day.