error C2440: “return”: 无法从“const Screen”转换为“Screen &”

转换丢失限定符。出错代码（例子来自c++ primer 4th）：Screen& Screen::display(std::ostream& os) const{ os << contents...

error C2440: 'initializing' : cannot convert from 'char *' to 'const class std::basic_string,class std::allocator > *'

error C2440: &#39;initializing&#39; : cannot convert from &#39;char *&#39; to &#39;const class std::basic_string<char,...