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. When something bad happens (like trying to open a file that does not exist), the stream simply goes into a failure state and refuses to do anything more. In the case of the program above, it never even tries to read in values when we do inFile >> input;. The variable just keeps its existing value.

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.fail()) {
  // 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. If there is an error, the code returns from main, which ends the program. (Returning a non-zero value is the convention for “there was an error in the program”). There is no point in trying to read data if we could not open the file! 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;
}