How to Read From a File in C++?

Reading from a file means retrieving the data stored inside a file as input in the program. In this article, we will learn how to read data from a file in C++.

There are two types of files: text files and binary files and the method of reading data from these files also differs from one another.

Read From a Text File

To read the content of a text file in C++, we have to first create an input file stream std::ifstream to the file in default flags. After that, we can use any input function such as std::getline() or >> operator to read the textual data and store it in some the string variable. We generally prefer getline() as it reads till the newline, or any specified character is found.

Code Implementation

// C++ program to illustrate how to read data
// from a text file
#include <bits/stdc++.h>
using namespace std;

int main() {

    // Open the text file named "input.txt"
    ifstream f("input.txt");

    // Check if the file is successfully opened
    if (!f.is_open()) {
        cerr << "Error opening the file!";
        return 1;
    }

    // String variable to store the read data
    string s;

    // Read each line of the file and print it to the
    // standard output stream till the whole file is
  	// completely read
    while (getline(f, s))
        cout << s << endl;

    // Close the file
    f.close();
    return 0;
}

input.txt

Hey Geek!
Welcome to GfG.
Happy Coding.

Output

Hey Geek!
Welcome to GfG.
Happy Coding.

Read From a Binary File

Binary files in C++ are used to store data in the binary form i.e. in the form of 0s and 1s. So, we need to open the input file stream ifstream object with the std::ios::binary flag. std::getline() function cannot read the data from the binary files so we use the specialized function ifstream::read() of ifstream class that reads the given block of data from the file stream and return it as a character array which can be converted to the required type using reinterprit_cast.

Syntax of ifstream::read()

stream.read(buffer, size)

where,

• stream: A valid ifstream object to binary file.
• buffer: Pointer to the character array where the read data is to be stored.
• size: Number of bytes to read.

Example

// C++ Program to illustrate how to read data
// from binary file
#include <bits/stdc++.h>
using namespace std;

// Structure type to store in the file
struct A {
    char name[50];
    int age;
};

int main() {
  
    // Opening the binary file
    ifstream f("input.bin", ios::binary);
    if (!f) {
        cout << "Unable to open file for" <<
          " reading\n";
        return 1;
    }
  
  	// Structure variable to store read data
    A st;
  
    // Read the struct data from binary file
  	// reinterpret_cast is used to convert the
  	// pointer of type st* to char*
    f.read(reinterpret_cast<char*>(&st),
           sizeof(st));

    cout << "Name: " << st.name << endl;
    cout << "Age: " << st.age << endl;
  
    f.close();
    return 0;
}

Output

Name: Anmol
Age: 24