构造函数不适用于从std :: string继承的类

#include<iostream>
#include<string>

class MyString : public std::string {
public:
    MyString(const std::string& str) : std::string(str) {}
};

int main() {
    MyString my_str("Hello, world!");
    std::cout << my_str<< std::endl;
    return 0;
}