Файлы C++

std::fstream file("iofile.txt", std::fstream::out | std::fstream::in);

file.seekg(0, std::fstream::beg);

std::fstream file("msg.txt", std::fstream::out | std::fstream::in);
 
if (!file.is_open())
        msg_exit("file not open()", EXIT_FAILURE);

#include <fstream>
#include <iostream>
#include <string>
 
 
 
int main() {
 
    //creates a file
    std::fstream file("iofile.txt", std::fstream::out);
 
    if (!file.is_open()) {
 
        std::cerr << "Can't open the file" << '\n';
        return 1;
 
    }
 
    file.close();
    file.open("iofile.txt", std::fstream::out | std::fstream::in);
 
    //write two lines
    file << "The first line\n";
    file << "The second line\n";
 
    file.seekg(0, std::fstream::beg);
 
    std::string str;
    std::getline(file, str, '\n');
 
    std::cout << str << '\n';
 
    return 0;
 
}

#include <iostream>
#include <fstream>
 
int main()
{
    std::fstream f("test.txt", f.app | f.in | f.out);
 
    f << 2021 << ' ';
 
    f.seekg(0, std::ios_base::beg);
 
    size_t n = -1; f >> n;
 
    std::cout << n;
}

    std::fstream file("ff", std::ios::app | std::fstream::out | std::fstream::in);
    if(file.is_open())
    {
        file << "somestr\n";
 
        file.seekg(0);
        std::string str;
        std::getline(file, str);
        std::cout << "from file: " << str;
 
        file.close();
    } else {
        std::cout << "OOps";
    }

std::ios::app | std::fstream::out | std::fstream::in