basic_string&
operator+=(const basic_string& str);
Effects: Calls append(str).
Returns: *this.
basic_string& operator+=(const charT* s);
Effects: Calls append(s).
Returns: *this.
basic_string& operator+=(charT c);
Effects: Calls push_back(c);
Returns: *this.
basic_string& operator+=(initializer_list<charT> il);
Effects: Calls append(il).
Returns: *this.