Reading and Writing to files in C++

The <fstream> header provides most functionalities we will need to write and read data from files. Don't forget to include it to use it's objects and functions.

Writing files

We use the ofstream object to write data to a file. You simply declare this variable then call its open function and pass the name of the file you want to store data in.

std::ofstream outputfile;
outputfile.open("output.txt");

You can use the insertion operator (<<) with the ofstream object to write data to the file. It's syntax is very similar to how we use cout. We are only using the name of the ofstream object instead; outputfile in this particular example.

outputfile << "Hello\n";

Don't forget to close the file to ensure that all data is stored properly.

outputfile.close();

You can try compiling and running output.cpp to see an example of how this works.

clang++ -std=c++17 output.cpp -o output
./output

After running the program, you will notice that it produces a new file called output.txt that contains the data we wrote in the program.

Reading files

We use the ifstream object to read data from a file. You simply declare this variable then call its open function and pass the name of the file you want to read data from.

std::ifstream inputfile;
inputfile.open("output.txt");

You can use the extraction operator (>>) with the ifstream object to read data from a file. It's syntax is very similar to how we use cin. We are only using the name of the ifstream object instead; inputfile in this particular example. The code below will extract the first string from the file and store it inside of the variable line. We can display its value using cout.

std::string line;
inputfile >> line;
std::cout << line;

Don't forget to close the file to ensure that the data does not get corrupted.

inputfile.close();

You can try compiling and running input.cpp to see an example of how this works.

This step assumes that you already ran output and created a file called output.txt. The program will throw an error if output.txt does not exist.

clang++ -std=c++17 input.cpp -o input
./input

Running the program will read data from the file, store it in varialbes, and display the values of those variables.

Working with arrays

Arrays are often useful when working with files. They allow us to store all values in an array then later write them to a file. Similarly, you can read values from a file and store them into an array.

Writing array values to files

The example below shows how we iterate through an array and write its contents into a file. If you notice, it looks almost exactly like displaying an array's values to the screen, but we write it to the output filestream instead.

std::ofstream outputfile;
outputfile.open("array_output.txt");

int arr_values[5] = {3, 6, 2, 1, 5};
outputfile << 5 << "\n"; // indicate how many elements will be saved
for (int i = 0; i < 5; i++)
  outputfile << arr_values[i] << "\n";

  outputfile.close();

You can try compiling and running array_output.cpp to see an example of how this works.

clang++ -std=c++17 array_output.cpp -o array_output
./array_output

After running the program, you will notice that it produces a new file called array_output.txt that contains the data we wrote in the program.

Storing values from files into arrays

The example below shows how we load data from a file and store it into an array. The limitation of arrays is that you need to define its size before using it. This example assumes that the first line of the data (output.txt) contains the number of elements and the rest are the values. However, not all files are formatted this way. If this is not the case then you will need to use a different method for loading values.

Also, take note of the data type the array contains. In the example below, arr_values is an integer array, so when we extract values from the inputfile, it knows that it should retrieve an integer value so it can be stored into an integer-sized container.

  std::ifstream inputfile;
  inputfile.open("array_output.txt");
  int size = 0;
  inputfile >> size; // identify the number of elements and use it to define the array size
                     // only works for this example, that assumes a size is provided before
                     // the values

  // Store data into array
  int arr_values[size];
  for (int i = 0; i < size; i++)
    inputfile >> arr_values[i];

  // Display data stored in the array
  for (int i = 0; i < size; i++)
    std::cout << arr_values[i] << "\n";
    
  inputfile.close();

You can try compiling and running array_input.cpp to see an example of how this works.

This step assumes that you already ran array_output and created a file called array_output.txt. The program will throw an error if array_output.txt does not exist.

clang++ -std=c++17 array_input.cpp -o array_input
./array_input