C++ Files

Disk instead of the console

cout writes to the screen. Files keep data after the program ends. In C++ you include<fstream>. ofstream writes. ifstream reads. Both are streams, so<< and >> look like cout and cin.

What the editor can still run

This program is a complete main. It compiles in the C++ editor. It does not need a file. Use it to confirm the chapter examples open in /cpp/try, then download a file example for a real disk.

#include <iostream>
using namespace std;

int main() {
  cout << "File streams need a disk." << endl;
  cout << "On StudyGrid, download the .cpp and compile with g++." << endl;
  return 0;
}

ofstream writes

Construct an ofstream with a path. If the file opens, send text with <<. If it does not, print an error and skip the write. Never assume the path worked.

#include <iostream>
#include <fstream>
using namespace std;

int main() {
  ofstream out("notes.txt");
  if (!out.is_open()) {
    cout << "could not open notes.txt for writing" << endl;
    return 0;
  }
  out << "first line" << endl;
  out << "second line" << endl;
  out.close();
  cout << "wrote notes.txt" << endl;
  return 0;
}

On a machine with a filesystem this creates notes.txt in the working directory. In/cpp/try you should see the “could not open” message. That is the check doing its job, not a broken example.

ifstream reads

ifstream opens an existing file. Check is_open() the same way. Then read line by line with getline, or token by token with >>.

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main() {
  ifstream in("notes.txt");
  if (!in.is_open()) {
    cout << "could not open notes.txt for reading" << endl;
    return 0;
  }
  string line;
  while (getline(in, line)) {
    cout << line << endl;
  }
  in.close();
  return 0;
}

Run this locally after the write example, in the same folder. The loop stops when there are no more lines. Closing is polite; the destructor also closes the file when in goes out of scope.

A stream that is not a file

stringstream lives in memory. It uses the same << and >> as a file, so you can practice stream code in the browser. Include <sstream>.

#include <iostream>
#include <sstream>
#include <string>
using namespace std;

int main() {
  stringstream ss;
  ss << "3 4";
  int a = 0;
  int b = 0;
  ss >> a >> b;
  cout << a + b << endl;
  return 0;
}

Output is 7. Same idea as reading two numbers from a file, without touching disk. When you are ready for a real file, swap stringstream for ifstream and keep the >> lines.

Open, check, then use

• Always test is_open() (or the stream in a boolean context) before reading or writing.
• A failed open is not a crash if you handle it. Using a closed stream is wasted work and empty reads.
• Paths are relative to the process working directory unless you pass an absolute path.

Local compile for real files

Save a .cpp from this page. In that folder run g++ -std=c++17 files.cpp -o files then./files (or files.exe on Windows). You should see notes.txt appear next to the program. Next: what to do when an operation cannot continue — exceptions.