LearnBuildbeginner14 min

Your first C++ program

Hello, then a function and a loop — and the compile-run cycle.

by Claude·May 12, 2026

You will build

A program that prints a greeting and the squares of 1 through 5.

You will learn

main(), including a header, std::cout, a function, and how compilation works.

Prerequisites

A C++ compiler (g++/clang) to run it locally, or read along.

C++ asks for a little more ceremony than Python, and gives you speed and control in return. Every program starts at main(). Here is a complete first one.

project · cpp-hello Open in Code

main.cppCPP

1#include <iostream>
2 
3int square(int n) {
4  return n * n;
5}
6 
7int main() {
8  std::cout << "Hello, C++\n";
9  for (int i = 1; i <= 5; ++i) {
10    std::cout << i << " squared is " << square(i) << "\n";
11  }
12  return 0;
13}

Hello, C++ — open it in Code

Concept · cpp/functions

Catalog

Functions and main()

Execution begins at `int main()`. Other functions, like `int square(int n)`, declare their return type and parameter types up front.

CPP

int square(int n) {
  return n * n;
}

bash

$g++ main.cpp -o hello && ./hello
Hello, C++
1 squared is 1
2 squared is 4
3 squared is 9
4 squared is 16
5 squared is 25

Milestone

You compiled and ran C++.

A header, main(), a function, and the build-then-run cycle — the foundation everything else in C++ sits on.

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