The Style Guide currently says:

  Use One-True-Brace-Style. The opening brace for a function or class
  is normally at the end of the line; it is sometimes moved to the
  beginning of the next line for emphasis. Substatements are enclosed
  in braces, even if there is only a single statement. Extremely
  simple one-line statements may drop braces around a substatement.

Why not just skip the last sentence? People interpret extremely simple one-liners differently and this sentence just adds a loop-hole for those that like to write if-statements without braces. This could be considered an extremely simple one-liner:

  int append(const E& elem) {
    if (this->_len == this->_max) grow(this->_len);
    int idx = this->_len++;
    this->_data[idx] = elem;
    return idx;
  }

but some people think we should not be writing code like this anymore.