Нужно пояснить работу с потоками ofstream, ifstream, fstream

#include <iostream>
#include <fstream>
 
const int MAX = 100;
 
void testifiles() {
    std::ifstream prog("../streams/streams.cpp");
    char line[MAX];
    for(int i = 0; i < 10; i++) {
        prog.getline(line, MAX);
        std::cout << line << std::endl;
    }
 
    std::ifstream exe("../streams/Debug/streams",
                     std::ios::in | std::ios::binary);
    char data[MAX];
    exe.read(data, MAX);
    for(int i = 0; i < MAX; i++)
        std::cout << std::hex << (int)data[i] << std::endl;
}
 
void testofiles() {
    std::ofstream email("email.txt");
    email << "Hi, there!" << std::endl << std::endl;
    email << "Thought you might be interested to know";
    email << " that the first " << MAX << " natural numbers are:" << std::endl;
    for(int i = 0; i < MAX; i++)
        email << i << ' ';
    email << "KTHXBYE!" << std::endl;
    email.close();
    std::clog << "Wrote file email.txt!\n";
    std::cout << "Press enter to continue...";
    std::cin.get();
 
    std::ofstream data("numbers.dat",
            std::ios::out | std::ios::binary);
    int numbers[MAX];
    for(int i = 0; i < MAX; i++)
        numbers[i] = i * 2;
    data.write((const char*)numbers, MAX * sizeof(int));
    std::clog << "Wrote file numbers.dat\n";
}
 
void testiofiles() {
    std::fstream data("numbers.dat",
                        std::ios::in | std::ios::out |
                        std::ios::binary);
    int numbers[MAX];
    for (int i = 0; i < 3; i++) {
        data.read((char*)numbers, 2 * sizeof(int));
        numbers[0] += numbers[1];
        data.write((const char*)numbers, 2 * sizeof(int));
    }
    std::streampos pos = data.tellp();
    data.seekp(-2 * sizeof(int), std::ios::end);
    numbers[0] = 100;
    numbers[1] = 200;
    data.write((char const*)numbers, 2 * sizeof(int));
    data.seekp(pos);
    data.write((char const*)numbers, 2 * sizeof(int));
    std::clog << "Modified file numbers.dat\n";
}
 
int main() {
    // testifiles();
    // testofiles();
    testiofiles();
    return 0;
}