(std::string const &,
 std::string const & abc,
 std::string const & = "abc",
 std::string const &)
(const std::string &,
 const std::string & abc,
 const std::string & = "abc",
 const std::string &)
 std::string const abc;
 std::string const abc = "abc";
 std::string const abc("abc");
 const std::string abc;
 const std::string abc = "abc";
 const std::string abc("abc");
