All functions to handle file are provdied by operating system. In modern operating system, common programs are forbidden to read and write disk direcctly.
Hence, file handling is an operation which requests operating system to provide a file object and common programs operate files via handling the file object.

Python

Create a File Object

To create a file object, we need to use the built-in open function. The open function returns the file object that contains methods and attributes to perform
various operations on the file.

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file_object = open("filename", "mode")

Here,

  • filename: gives name of the file that the file object has opened.
  • mode: attribute of a file object tells you which mode a file was opened in.

Read a File

Use the open function and gives the r attribute in mode as a flag, we could open a file in read mode.

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f = open("/Users/Carlos/test.txt", "r")

Note that open function will throw an IoError if the file does not exist and print the detailed information about errors.

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Traceback (most recent call last):
	File "<stdin>", line 1 in <module>
FileNotFoundError: [Errno 2] No such file or directory: '/Users/Carlos/test.txt'

if open the file successfully, use f.read to read file datat and store it in variable content.

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contents = f.read()

Finally, use f.close to close the file and release system resources. Note that each file should be closed correctly after being operated.

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f.close()

However, if IOError occurs while reading or writing the file object, the following f.close function will not be called. In order to close the file correctly,
using with syntax block to open an file could gurantee that close function will be called whatever happens:

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with open('/Users/Carlos/test.txt', 'r') as f:
    contents = f.read()

Read a File line by line

The read function may lead to memory leak since it will read and save all the contents of the file. Therefore, we could use another function f.readlines() or f.read(size) to read contents in fixed size.

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with open('/User/Carlos/test.txt', 'r') as f:
    lines = f.readlines()
    for line in lines:
        print(line)

Write a File

Use the same open function and gives the w attribute in mode as a flag, wec could opend a file in write mode.

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f = open('/Users/Carlos/test.txt', 'w')

After opening a file object, f.write function could be called repeatedly to write contents into the file. Note that the file object have been closed correctly through f.close function after all contents being written in it.

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f.write('Hello, world!')
f.close()

with block also could be use in w mode:

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with open('/User/Carlos/test.txt', 'w') as f:
    f.write('Hello, world!')

Append Data to a File

whatever we write will cover the existing contents in the file if we use w mode to open a file object. Hence we could use a mode to add a new text to the already existing file.

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f = open("/Users/Carlos/test.txt", "a+"):
f.write('Hello world!')
f.close()

Files Modes in Python

Mode Description
‘r’ This is the default mode. It opens file for reading.
‘w’ This Mode opens file for writing
If file does not exist, it creates a new files.
If file exists it truncates the file.
‘x’ Creates a new file. If file already exists, the operation fails.
‘a’ Open file in append mode.
if file does not exist, it creates a new file.
‘t’ This is the default mode. It opens in text mode.
‘b’ This opens in binary mode.
‘+’ This will open a file for reading and writing.

C++

Standard Library

In C++, the fstream standard library allows us to work with files.

To use the fstream library, include both the standard <iostream> and the <fstream> header file:

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#include <iostream>
#include <fstream>

Write a File

To create a file, use either the ofstream and fstream object, and specify the name of the file.

To write to the file, use the insertion operator (<<).

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#include <iostream>
#include <fstream>

using namespace std;

int main() {
    // Create and open a text file
    ofstream MyFile("filename.txt");

    // Write to the file
    MyFile << "Files can be tricky, but it is fun enough!";

    // Close the file
    MyFile.close();
}

Close the file is a good practice, and it can clean up unnecessary memory space.

Read a file

To read from a file, use either the ifstream or fstream object, and the name of the file.

Note that we also use a while loop together with the getline() function or other functions.

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#include <iostream>
#include <fstream>

using namespace std;

int main()
{
  // Create a text string, which is used to output the text file
  string myText;

  // Read from the text file
  ifstream MyReadFile("filename.txt");

  // Use a while loop together with the getline() function to read the file line by line
  while (getline (MyReadFile, myText)) {
    // Output the text from the file
    cout << myText;
  }
  // Close the file
  MyReadFile.close();
}

References