File I/O in C++

What is File I/O?

File I/O (Input/Output) allows your C++ programs to read data from files and write data to files. This is useful for saving data between program runs.

Sequential Access Files

Sequential access files (also called text files) store lines of text that your program can read in order, from beginning to end. They can be either:

Input files – files the program reads from
Output files – files the program writes to

Required Header


#include <fstream>

This header gives access to:

ifstream – for input file objects
ofstream – for output file objects
fstream – for files that do both

Opening Files

Use the open() function to connect a file object to a file:


ofstream outFile;
outFile.open("foods.txt");

You can check if it worked using is_open():


if (outFile.is_open()) {
    // success
}

Writing to Files

Each line written can represent a record. Records can contain multiple fields, separated by a special character like #.


outFile << "Pizza#9.99" << endl;
outFile << "Tacos#7.49" << endl;

Reading from Files

You can read full lines using getline(), or use >> to read word by word or field by field. Use eof() to detect the end of the file:


while (!inFile.eof()) {
    getline(inFile, line);
    // process line
}

Don't Forget!

✔️ Always close your files with close() when done.
✔️ Each line (record) should end with endl.
✔️ Use characters like # to split up fields within a record.

How File Streams Compare to `cin` and `cout`

cout << "Pizza"; – prints to the console
outFile << "Pizza"; – writes to a file
cin >> name; – reads from the user
inFile >> name; – reads from a file

Code Walkthrough – Full Example

This program saves a list of foods and their prices to a file, then reads and displays them:


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

int main() {
    ofstream outFile("foods.txt");

    if (!outFile.is_open()) {
        cout << "Could not open file for writing." << endl;
        return 1;
    }

    outFile << "Pizza#9.99" << endl;
    outFile << "Burger#5.49" << endl;
    outFile << "Ice Cream#3.25" << endl;
    outFile.close();

    ifstream inFile("foods.txt");
    string line;

    if (!inFile.is_open()) {
        cout << "Could not open file for reading." << endl;
        return 1;
    }

    cout << "Food Menu:\n";
    while (getline(inFile, line)) {
        size_t separator = line.find('#');
        string name = line.substr(0, separator);
        string price = line.substr(separator + 1);
        cout << "- " << name << ": $" << price << endl;
    }

    inFile.close();
    return 0;
}

Key Terms

Field – A single piece of info (e.g. price)
Record – A full set of related fields (e.g. Pizza + price)
Input file – A file your program reads from
Output file – A file your program writes to
open() – Opens a file
is_open() – Checks if file opened successfully
close() – Closes a file
eof() – True if at end of file
Sequential access file – Text file read from start to end
Text file – Another name for sequential file
not logical operator (!) – Reverses a condition’s truth
Scope resolution operator (::) – Links function/member to class

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