c++ 把istream转为ifstream

代码例子：

#include <iostream>
#include <sstream>
#include <fstream>

int main() {
    std::string a = "Hello, World!";
    std::istringstream ss;
    ss.str(a);
    std::istream &iss(ss);
    std::ifstream &fs = static_cast<std::ifstream &>(iss);
    std::string b;
    while (!fs.eof()) {
        fs >> b;
        std::cout << b << std::endl;
    }
    return 0;
}

输出：

Hello,
World!

Process finished with exit code 0