<fstream> gives you std::ofstream (write), std::ifstream (read) and std::fstream (both). They work exactly like cin/cout and close themselves (RAII).

Write, then read back

C++

#include <cstdio>
#include <fstream>
#include <iostream>
#include <string>

int main() {
    {
        std::ofstream out("scores.txt");                 // truncates / creates
        if (!out) { std::cerr << "cannot open\n"; return 1; }
        out << "ada 91\n" << "bob 78\n" << "cy 85\n";
    }                                                     // closed (and flushed) here

    {
        std::ofstream app("scores.txt", std::ios::app);  // append mode
        app << "dee 99\n";
    }

    std::ifstream in("scores.txt");
    std::string name;
    int score, total = 0, count = 0;
    while (in >> name >> score) {                         // token-by-token
        total += score;
        ++count;
    }
    std::cout << count << " students, average " << total / count << '\n';

    in.clear();                                           // reset EOF flag…
    in.seekg(0);                                          // …and rewind
    std::string line;
    std::getline(in, line);                               // line-by-line
    std::cout << "first line: " << line << '\n';
    in.close();
    std::remove("scores.txt");
}

Output

4 students, average 88
first line: ada 91

Mode flag	Meaning
`std::ios::in` / `std::ios::out`	Read / write (defaults for `ifstream` / `ofstream`)
`std::ios::app`	Every write goes to the end
`std::ios::trunc`	Discard existing contents (default for `ofstream`)
`std::ios::binary`	No newline translation (matters on Windows)
`std::ios::ate`	Start at the end, but may seek anywhere

Stream state

Check	True when
`if (stream)` / `!stream.fail()`	Last operation succeeded
`stream.eof()`	Tried to read past the end
`stream.fail()`	Format error or EOF (e.g. reading letters into an `int`)
`stream.bad()`	Unrecoverable I/O error
`stream.clear()`	Resets the flags so you can continue

Binary files & whole-file reads

C++

#include <cstdio>
#include <fstream>
#include <iostream>
#include <sstream>
#include <vector>

int main() {
    std::vector<int> data{1, 2, 3, 1000000};
    {
        std::ofstream out("data.bin", std::ios::binary);
        out.write(reinterpret_cast<const char*>(data.data()), data.size() * sizeof(int));
    }
    std::ifstream in("data.bin", std::ios::binary);
    std::vector<int> back(4);
    in.read(reinterpret_cast<char*>(back.data()), back.size() * sizeof(int));
    std::cout << in.gcount() << " bytes, last = " << back[3] << '\n';

    std::ofstream("note.txt") << "line one\nline two\n";
    std::ifstream t("note.txt");
    std::stringstream buffer;
    buffer << t.rdbuf();                                  // slurp the whole file
    std::cout << buffer.str().size() << " chars\n";
    std::remove("data.bin");
    std::remove("note.txt");
}

Output

16 bytes, last = 1000000
18 chars

Do you need to call close() on an fstream?