Welcome to Programming and Data Structures

In C++ we include the <fstream> library to work with files. It provides two key types of object: ifstream (input file stream) and ofstream (output file stream). These objects are used to read from and write to files respectively.

To read from a file, we need to create a std::ifstream object. (If you are using namespace std; you can shorten that to ifstream). Like making a string or any other variable, we need to give the stream a name:

ifstream inFile;

Once we have a stream object, we need to open a file. This is done with the open method of the stream object. The open method takes a string as a parameter that is the name of the file we want to open. Once the file is open, we can read from it using the >> operator just like we were reading from the console.

This program opens the datafile Numbers.txt shown on the previous page and reads the first two numbers from it:

The streams provided by <fstream> are a bit frustrating in that they do not automatically report errors. If you want to know if the stream is OK, you have to use one of these checks:

// Check if the stream is OK
if (inFile.good()) {
  // stream is OK
}
// Or short version:
if (inFile) {
  // stream is OK
}

// Check for errors
if (inFile.error()) {
  // stream is NOT OK
}
// Or short version:
if (!inFile) {
  // stream is NOT OK
}

Here is a version of the program that checks to make sure the file was opened successfully. It also uses a shortened form of creating the ifstream and opening a file with it.

Try changing the filename this program is opening to see that it catches the issue. By returning from main, we end the program early when there was a failure opening the file. (Returning a non-zero value is the convention for “there was an error in the program”). We could instead throw an exception, try opening a different file, etc...

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

int main() {
  string filename = "input.txt";
  ifstream infile;
  infile.open(filename);
  string input;
  filename >> input;
}