Basic Input / Output in C++

In C++, data is read and written using streams, which are sequences of bytes.

• Input stream: Data flows from a device (like the keyboard) to the computer’s memory.
• Output stream: Data flows from memory to an output device (like the screen).
• These streams are defined in the <iostream> header file.
• The most common stream objects are - "cin": for taking input and "cout": for displaying output.

Standard Output Stream - cout

• cout is an instance of the ostream class used to display output on the screen.
• Data is sent to cout using the insertion operator <<.

#include <iostream>
using namespace std;

int main()
{
    cout << "GeeksforGeeks";
    return 0;
}

GeeksforGeeks

Standard Input Stream - cin

• cin is an instance of the istream class used to read input from the keyboard.
• The extraction operator >> is used with cin to get data from the input stream and store it in a variable.

#include <iostream>
using namespace std;

int main()
{
    int age;
    // Taking input from user and store it in variable
    cin >> age;
    
    // Output the entered age
    cout << "Age entered: " << age;
    return 0;
}

Output

18 (Enter by user)
Age entered: 18

Un-buffered Standard Error Stream - cerr

• cerr is the standard error stream used to display error messages. It is an instance of the ostream class.
• It is an unbuffered output stream used to display error or warning messages immediately, ensuring they appear instantly without any buffering delays like cout

Note: cerr is unbuffered, so its output appears immediately, while cout is buffered and may be delayed. Both can be redirected to files unbuffered does not prevent file output.

#include <iostream>
using namespace std;

int main()
{
    cerr << "An error occurred";
    return 0;
}

Error

An error occurred

Buffered Standard Error Stream - clog

• clog is the standard logging stream used to display error or log messages. It is an instance of the ostream class, like cerr.
• Messages are first stored in a buffer and displayed only when the buffer is full or explicitly flushed using flush() (Buffered output)
• Useful for logging messages that don’t need to appear immediately on the screen.
• Unlike cerr, output from clog may be delayed due to buffering.

#include <iostream>
using namespace std;

int main()
{
    clog << "An error occurred";
    return 0;
}

Error

An error occurred

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Difference between cerr and clog