IntermediatePractice in the editor

C++ Files (fstream)

The header fstream provides file streams that work like cout and cin: std::ofstream writes a file, std::ifstream reads one and std::fstream can do both. Files your program creates appear in the editor's file list.

Writing a file

ExampleTry it in the editor

#include <fstream>
#include <iostream>

int main() {
    std::ofstream out("marks.txt");          // create, or empty if it exists
    if (!out) {
        std::cout << "Could not create marks.txt\n";
        return 1;
    }
    out << "Asha 91\n";
    out << "Ravi 78\n";
    out << "Meera 85\n";
    std::cout << "written\n";
    return 0;                                // out closes itself here
}

Output

written

A stream converts to false when something went wrong, so if (!out) checks that the file opened. The file is closed automatically when the stream object goes away (RAII); out.close() closes it earlier if needed.

Reading values

ExampleTry it in the editor

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

int main() {
    std::ofstream("marks.txt") << "Asha 91\nRavi 78\nMeera 85\n";

    std::ifstream in("marks.txt");
    if (!in) {
        std::cout << "Could not open marks.txt\n";
        return 1;
    }
    std::string name;
    int mark, total = 0, count = 0;
    while (in >> name >> mark) {             // stops at the end of the file
        std::cout << name << " scored " << mark << "\n";
        total += mark;
        count++;
    }
    std::cout << "Average: " << static_cast<double>(total) / count << "\n";
    return 0;
}

Output

Asha scored 91
Ravi scored 78
Meera scored 85
Average: 84.6667

The loop condition is the read itself, so it stops cleanly at the end of the file. Avoid while (!in.eof()), which runs one time too many.

Line by line

ExampleTry it in the editor

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

int main() {
    {
        std::ofstream out("poem.txt");
        out << "Roses are red,\nviolets are blue,\nC++ is fast,\nand so are you.\n";
    }
    std::ifstream in("poem.txt");
    std::string line;
    int n = 0;
    while (std::getline(in, line)) {
        std::cout << ++n << ": " << line << "\n";
    }
    return 0;
}

Output

1: Roses are red,
2: violets are blue,
3: C++ is fast,
4: and so are you.

Appending

Open with std::ios::app to add at the end instead of starting over:

ExampleTry it in the editor

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

int main() {
    for (int run = 1; run <= 3; run++) {
        std::ofstream log("log.txt", std::ios::app);
        log << "entry " << run << "\n";
    }
    std::ifstream in("log.txt");
    std::string line;
    while (std::getline(in, line)) std::cout << line << "\n";
    return 0;
}

Output

entry 1
entry 2
entry 3

Splitting a line

std::istringstream (header sstream) reads values from a string, which is handy for lines in a fixed format such as comma-separated values: std::getline(row, field, ',') reads up to the next comma.

Common mistakes

Forgetting to check that the file opened. A misspelled name gives an empty read with no error message unless you test the stream.
Mixing in >> x and getline: after >> the newline stays in the stream, and the next getline returns an empty line. Call in.ignore() in between, or read everything with getline.
Opening an ofstream on a file you meant to keep: without std::ios::app its contents are erased.

Practice exercises

Write the names of five students to class.txt, one per line, then read the file and count the names.
Copy poem.txt into copy.txt line by line.
Read a file of name,mark lines with getline(..., ',') and print the best student.