如何将文件内容读入istringstream？

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

std::ifstream file("file_name.txt");

if (!file.is_open()) {
    std::cerr << "Error opening file"<< std::endl;
    return 1;
}

std::stringstream buffer;
buffer<< file.rdbuf();

file.close();

std::string line;
while (std::getline(buffer, line)) {
    std::cout<< line<< std::endl;
}

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

int main() {
    std::ifstream file("file_name.txt");

    if (!file.is_open()) {
        std::cerr << "Error opening file"<< std::endl;
        return 1;
    }

    std::stringstream buffer;
    buffer<< file.rdbuf();
    file.close();

    std::string line;
    while (std::getline(buffer, line)) {
        std::cout<< line<< std::endl;
    }

    return 0;
}