Input and Output in C++

Input means receiving data from the user or another input source. For example, a program can ask the user to enter their age.

int age;

std::cin >> age;

Here, the value entered by the user is stored in age.

Output means displaying information produced by a program.

std::cout << "Hello World";

This statement displays text on the standard output stream.

The <iostream> header provides facilities for standard input and output.

#include <iostream>

It is commonly included when using std::cin and std::cout.

std::cout is used to send output to the standard output stream.

#include <iostream>

int main() {

    std::cout << "Welcome to C++";

    return 0;
}

Text can be displayed by placing it inside double quotation marks.

std::cout << "Hello Students";

The text inside the quotation marks is displayed as output.

Variables can also be displayed using std::cout.

int age = 20;

std::cout << age;

The value stored in age is displayed.

The << operator is used with std::cout to send data to the output stream.

std::cout << "Age: " << age;

Multiple pieces of data can be sent to the output stream in one statement.

int age = 20;
double marks = 85.5;

std::cout << "Age: " << age;
std::cout << " Marks: " << marks;

The insertion operator can be chained to display multiple values.

std::endl inserts a newline and flushes the output stream.

std::cout << "Hello" << std::endl;
std::cout << "Welcome";

The newline escape sequence \n can also be used to move output to the next line.

std::cout << "Hello\n";
std::cout << "Welcome";

Unlike std::endl, writing \n does not itself request a flush.

std::cin is commonly used to read formatted input from the standard input stream.

int age;

std::cin >> age;

The entered value is stored in age.

The >> operator is used with std::cin to extract formatted input.

int number;

std::cin >> number;

The input is converted to the type of the destination variable when the formatted extraction succeeds.

#include <iostream>

int main() {

    int age;

    std::cout << "Enter your age: ";
    std::cin >> age;

    std::cout << "Your age is " << age;

    return 0;
}

This program reads an integer and then displays it.

Multiple variables can be read using multiple extraction operations.

int age;
double marks;

std::cin >> age;
std::cin >> marks;

The user can enter the values separately or, for formatted extraction, provide them separated by whitespace.

int age;
double marks;

std::cin >> age >> marks;

The extraction operator can be chained to read multiple values.

char grade;

std::cout << "Enter grade: ";
std::cin >> grade;

std::cout << "Grade: " << grade;

The character entered by the user is stored in grade.

For a std::string, std::cin normally reads one whitespace-delimited word.

#include <iostream>
#include <string>

int main() {

    std::string name;

    std::cout << "Enter your name: ";
    std::cin >> name;

    std::cout << "Hello " << name;

    return 0;
}

Use std::getline() when you want to read an entire line, including spaces.

#include <iostream>
#include <string>

int main() {

    std::string fullName;

    std::cout << "Enter your full name: ";
    std::getline(std::cin, fullName);

    std::cout << "Name: " << fullName;

    return 0;
}

When std::getline() is used after formatted extraction such as std::cin >> age, a leftover newline may be read immediately.

int age;
std::string name;

std::cin >> age;
std::getline(std::cin >> std::ws, name);

Using std::ws here consumes leading whitespace before reading the line.

Escape sequences are special character combinations used inside string or character literals.

std::cout << "Hello\nWorld";
std::cout << "Name:\tRahul";

Common examples include \n for a newline and \t for a horizontal tab.

Boolean values can be printed directly.

bool passed = true;

std::cout << passed;

By default, a Boolean value is typically displayed as 1 for true and 0 for false.

The std::boolalpha stream manipulator can be used to display Boolean values as true and false.

#include <iostream>

int main() {

    bool passed = true;

    std::cout << std::boolalpha << passed;

    return 0;
}

std::cerr is the standard error stream and is commonly used for error messages.

#include <iostream>

int main() {

    std::cerr << "An error occurred.";

    return 0;
}

std::clog is the standard logging stream. It can be used for diagnostic or logging messages.

#include <iostream>

int main() {

    std::clog << "Program started.";

    return 0;
}

#include <iostream>

int main() {

    std::string name;
    int age;

    std::cout << "Enter your name: ";
    std::cin >> name;

    std::cout << "Enter your age: ";
    std::cin >> age;

    std::cout << "Name: " << name << std::endl;
    std::cout << "Age: " << age << std::endl;

    return 0;
}

Formatted input can fail if the entered data cannot be converted to the requested type.

int age;

std::cin >> age;

If the user enters invalid input such as non-numeric text when an integer is expected, the stream can enter a failure state.

The state of an input stream can be checked using it in a condition.

int age;

if (std::cin >> age) {
    std::cout << "Valid input";
} else {
    std::cout << "Invalid input";
}

This allows a program to respond when input extraction fails.

#include <iostream>
#include <string>

int main() {

    std::string name;
    int age;
    double marks;

    std::cout << "Enter your name: ";
    std::cin >> name;

    std::cout << "Enter your age: ";
    std::cin >> age;

    std::cout << "Enter your marks: ";
    std::cin >> marks;

    std::cout << "\n--- Student Details ---\n";
    std::cout << "Name: " << name << std::endl;
    std::cout << "Age: " << age << std::endl;
    std::cout << "Marks: " << marks << std::endl;

    return 0;
}

• Include <iostream> when using standard streams.
• Use std::cout for normal console output.
• Use std::cin for formatted console input.
• Use std::getline() when an entire line is required.
• Use std::cerr for error messages.
• Check input operations when invalid input is possible.
• Use meaningful prompts so users know what to enter.