gcc error: invalid use of member (did you forget the `&' ?)

Every time I have gotten the above compilation error, it has nothing to do with a missing ampersand. Instead, it is missing function-call parentheses:

  if (you_go && use->yourMember) { // WRONG!
    this->isInvalid();
  }

  if (i_go && use->myMember()) { // Oh yeah, the parens.
    continue;
  }

So don't use your member invalidly.