Introduction to C++ · Lesson 2 of 33

Your First C++ Program: iostream and Namespaces

5 min read Updated Nov 7, 2026 Saurabh Samir

On this page

Set up a compiler
Hello, World!
Output with cout
Input with cin
Namespaces
Practice
Summary

This lesson sets up a compiler, writes your first C++ program and explains every line. You'll print with std::cout, read input with std::cin, and learn what std:: and namespaces are – including the honest truth about using namespace std;.

In this lesson

Installing and running g++ or clang++
Hello World explained
Output with cout and input with cin
Namespaces and using

Set up a compiler

System	How
Windows	MSYS2 (`pacman -S mingw-w64-ucrt-x86_64-gcc`), or WSL with Ubuntu
macOS	`xcode-select --install` (clang++), or `brew install gcc` for g++
Ubuntu / Debian	`sudo apt install build-essential`

g++ -std=c++20 -Wall -Wextra hello.cpp -o hello
./hello

Hello, World!

#include <iostream>

int main() {
    std::cout << "Hello, World!\n";
    std::cout << "C++ version: " << __cplusplus << '\n';
    return 0;
}

Output

Hello, World!
C++ version: 202002

Line by line

#include <iostream>the input/output stream library (no .h in C++ headers)
int main()where the program starts; returning 0 means success
std::coutthe standard output stream; std:: is its namespace
<<"insert into the stream" – chain as many as you like
__cplusplusa number telling you which standard you're compiling with (202002 = C++20)

Output with cout

#include <iomanip>
#include <iostream>

int main() {
    int qty = 3;
    double price = 249.5;
    std::cout << "Qty: " << qty << ", price: " << price << '\n';
    std::cout << std::fixed << std::setprecision(2);          // 2 decimals from now on
    std::cout << "Total: " << qty * price << '\n';
    std::cout << std::setw(8) << "Item" << std::setw(8) << "Cost" << '\n';
    std::cout << std::setw(8) << "Pen" << std::setw(8) << 20.0 << '\n';
    std::cout << std::boolalpha << (qty > 2) << '\n';       // prints true instead of 1
    return 0;
}

Output

Qty: 3, price: 249.5
Total: 748.50
    Item    Cost
     Pen   20.00
true

cout knows the type of each value, so there are no format specifiers to get wrong (unlike printf). Use '\n' for new lines; std::endl also flushes the output, which is slower in loops.

Input with cin

#include <iostream>
#include <string>

int main() {
    std::string name;
    int age;
    std::cout << "First name: ";
    std::cin >> name;                       // reads one word
    std::cout << "Age: ";
    std::cin >> age;

    std::cin.ignore();                      // drop the newline left after the number
    std::string city;
    std::cout << "City (can have spaces): ";
    std::getline(std::cin, city);           // reads a whole line

    std::cout << name << " (" << age << ") lives in " << city << '\n';
    return 0;
}

Output

First name: Asha
Age: 21
City (can have spaces): New Delhi
Asha (21) lives in New Delhi

cin >> then getline

cin >> age leaves the Enter key's newline in the input. A following getline reads that empty remainder and returns immediately. Call std::cin.ignore() (or std::cin >> std::ws) first.

Namespaces

A namespace groups names so that libraries don't clash: the standard library lives in std. You can write your own:

#include <iostream>

namespace physics {
    double speed(double distance, double time) { return distance / time; }
}
namespace finance {
    double speed(double money, double days) { return money / days; }   // same name, no clash
}

int main() {
    std::cout << physics::speed(100, 9.58) << " m/s\n";
    std::cout << finance::speed(3000, 30) << " rupees/day\n";

    using std::cout;            // bring in one name
    cout << "only cout was imported\n";
    return 0;
}

Output

10.4384 m/s
100 rupees/day
only cout was imported

About using namespace std;

It imports every name from std – hundreds of them – into your code, which can clash with your own names (count, distance, max…). It's fine in a short contest solution or a single .cpp file, but never put it in a header file. This course writes std::; you'll see using namespace std; in competitive-programming examples, where speed of typing matters.

Practice

Try it yourself

Read two numbers and print their sum, difference, product and average (2 decimals).

Show solution

#include <iomanip>
#include <iostream>

int main() {
    int a, b;
    std::cout << "Two numbers: ";
    std::cin >> a >> b;
    std::cout << "sum " << a + b << ", difference " << a - b << ", product " << a * b << '\n';
    std::cout << std::fixed << std::setprecision(2) << "average " << (a + b) / 2.0 << '\n';
    return 0;
}

Output

Two numbers: 17 8
sum 25, difference 9, product 136
average 12.50

Quick check

What does std:: mean?

The name belongs to the std namespace – the standard library.

How do you read a full line containing spaces?

std::getline(std::cin, s).

'\n' vs std::endl?

Both end the line; endl also flushes the stream, which is slower.

Summary

Compile with g++ -std=c++20 -Wall -Wextra file.cpp -o name.
Print with std::cout <<, read with std::cin >> and std::getline.
Namespaces prevent name clashes; avoid using namespace std; outside small programs.

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