C++ istream

The C++ istream class provides a set of functions and operators for reading data from various input sources.

Before using the istream class, we need to include the <iostream> header in our program.

#include <iostream>

Example 1: Reading Data From the Console

To read data from the console using istream, you can use the cin object with the extraction operator >>. Here's an example:

#include <iostream>
using namespace std;

int main() {

    int entered_number;

    cout << "Enter an integer: ";

    // read data from the console
    cin >> entered_number;
    
    cout << "You entered: " << entered_number << endl;
    
    return 0;
}

Output

Enter an integer: 3
You entered: 3

In this example, cin represents the standard input stream that reads data from the keyboard. We use the >> operator to extract the user's input and store it in the number variable.

The get() Function

The get() function of the istream class is primarily used for reading individual characters from the input stream.

Key Characteristics

• Reading Characters: It reads the next character from the input stream and advances the stream's internal position indicator.

• Single Character: Unlike the >> operator, get() reads characters as they are, including spaces, tabs, and newline characters.

• Character Extraction: You can use it to extract characters into variables of type char or to store them in character arrays (C-strings).

• Delimiter Control: It allows you to specify a delimiter character as an optional parameter. When you specify a delimiter, get() stops reading characters when it encounters that delimiter. This is useful for reading words or lines.

Example 2: C++ get() Function

#include <iostream>
using namespace std; 

int main() {

    char ch;
    
    cout << "Enter a sentence: ";

    // read the character one by one
    // until a new line is encountered
    while (cin.get(ch) && ch != '\n') {
        cout << ch;
    }

    return 0;
}

Output

Enter a sentence: Hello World
Hello World

Here, we have used cin.get() to read a single character from the user.

Notice this part of the code:

while (cin.get(ch) && ch != '\n') {
    cout << ch;
}

The loop reads characters one by one from standard input until a newline character is encountered. In each iteration, it prints the character immediately as it is read.

C++ getline() Function

The getline() function in istream is used to read a line of text from the specified input stream (such as cin for standard input).

Key Characteristics

• Reading Lines: Unlike theget() function, getline() reads entire lines of text, up to a specified delimiter or the end of the line.

• Delimiter Control: You can specify a delimiter character like one that indicates the end of a line. This allows you to read multiple lines of text sequentially.

• Buffer Size: It accepts an optional parameter to specify the maximum number of characters to read, preventing buffer overflows and handling long lines gracefully.

Example 3: C++ getline() Function

#include <iostream>
using namespace std; 

int main() {

    string line;
    
    cout << "Enter a line of text: ";

    // read a line of input from user
    getline(cin, line);
    
    cout << "You entered: " << line << endl;

    return 0;
}

Output

Enter a line of text: Hello
You entered: Hello

C++ ignore() Function

The ignore() function is used to skip a given length of characters in the input stream.

#include <iostream>
using namespace std;

int main() {

    cout << "Enter a line of text: ";

    char ch;

    // ignore the next 5 characters from standard input
    cin.ignore(5);

    // read the sixth character and print the value
    if (cin.get(ch)) {
        cout << "Next character after ignoring 5 characters: " << ch << endl;
    }

    return 0;
}

Output

Enter a line of text: 12345678
Next character after ignoring 5 characters: 6

Here, cin.ignore(5); is instructing the input stream to ignore the next five characters that are entered by the user.

After this, the if condition cin.get(ch) attempts to read the next character from the input stream after the previous five have been ignored. If successful, it stores them in the ch variable.