https://blog.faceutil.com/2017/04/19/stdstring-%ea%b0%84%eb%8b%a8%ed%9e%88-%ec%86%8d%eb%8f%84-%ed%85%8c%ec%8a%a4%ed%8a%b8%eb%a5%bc-%ed%95%b4%eb%b3%b4%ec%95%98%eb%8b%a4-%c2%a7/