I like functions that return a value to be written in the guard-clause style. [In C++, that's probably better for efficiency since the compiler optimizer doesn't have to remove temporaries by converting the code to guard-clause style.]

However, I'm not convinced that void functions are better written in the guard-clause style. I think the intent of the code is better expressed with nested ifs. Of course, try to factor the ifs so that they're readable.

I'm saying this is more readable if the "do something part" is less than about 15 lines. Thoughts?

    void do_something(input) {
      if (!input.already_done()) {
        // do something.
      }
    }