"hello" is not an std::string. There are also quite a few leftover string classes in C++ from the olden daze, and people still cannot resist the urge to roll their own.
You can use "hello"s to get std::string in recent editions. There's a lot of type deduction going on so if one parameter is std::string the c string literals will be handled too: e.g: "hello"+"world"s should yield std::string. (At least in theory)
