Hello World and the Compilation Model

What is a program?

A program is a sequence of instructions that tells a computer exactly what to do, one step at a time. Every action your computer takes — drawing a window, sorting a list, sending a network packet — is ultimately the result of some program running. The CPU itself can only execute instructions in machine code: binary numbers that encode operations at the hardware level. Machine code is not readable by humans, and it is different for every CPU architecture (Intel x86, ARM, RISC-V, and so on).

C++ is a high-level programming language. You write instructions using words and symbols that make sense to humans, and a special program called a compilertranslates that text into machine code for your specific CPU. You write source code once; the compiler produces an executable that the hardware can run directly, at full speed — with no interpreter sitting in between at run-time.

C++ was designed by Bjarne Stroustrup in the early 1980s as an extension of the C language. It adds classes, objects, and many other features while keeping C's ability to generate highly efficient code. Today it powers operating systems, game engines, databases, browsers, and real-time systems where performance cannot be compromised.

Your first program

Create a file called hello.cpp (the .cpp extension identifies C++ source files) and type the following exactly:

#include <iostream>
using namespace std;

int main()
{
    cout << "Hello, World!" << endl;
    return 0;
}

Every line of this program is important. Here is what each piece does:

The curly braces { } delimit the body of the function — the block of statements that run when main is called. Every opening brace must eventually be paired with a closing brace.

The compilation model

Source code is plain text. Your CPU cannot run plain text — it can only execute binary machine instructions. The journey from .cpp file to running program involves two distinct programs: the compiler and the linker.

In practice, build systems (including the compiler command itself) typically run the compile and link steps back-to-back so you experience them as one action. But they are logically separate: the compiler works on a single source file; the linker assembles multiple object files into one program.

Compiling and running from the command line

The most direct way to compile is with the g++ command (or clang++ on macOS). Open a terminal in the directory containing hello.cpp and run:

# Compile — creates an executable called "hello" (Linux/macOS)
g++ -std=c++17 -Wall -o hello hello.cpp

# Run it
./hello

The flags matter:

On Windows with MSVC, the command looks different: cl /std:c++17 /W4 hello.cpp. Most developers on Windows use an IDE like Visual Studio or Visual Studio Code, which invokes the compiler for you through CMake — covered in the next lesson.

Printing special characters — escape sequences

Some characters cannot be typed directly inside a string literal. A double-quote would end the string prematurely; a backslash would be interpreted as the start of an escape sequence. C++ uses a backslash followed by a letter to represent these characters:

cout << "She said \"Hello.\"" << endl;  // prints: She said "Hello."
cout << "C:\\Users\\Alice"  << endl;  // prints: C:\Users\Alice
cout << "Line one\nLine two"  << endl;  // prints on two lines

Note that endl and "\n" both produce a newline, but endl also flushes the output buffer, which is slightly slower. For tight loops, prefer "\n".

Compile-time errors vs run-time errors

Mistakes in a program fall into two categories depending on when they are caught. Compile-time errors are caught by the compiler before the program ever runs — typically because of a syntax violation. Run-time errors (also called logic errors) slip past the compiler and only become visible when the program is actually executing, often producing wrong output or a crash.

// Error: missing semicolon
cout << "Hello, World!" << endl   // ← compiler error here
return 0;

// Error: missing closing brace
int main()
{
    cout << "Hello" << endl;
    // forgot to close with }

// Error: misspelled keyword or name
Int main()      // 'Int' is not a keyword — C++ is case-sensitive
    cout < "Hello";  // wrong operator, should be <<

// Compiles fine, but the math is wrong:
double area = width + height;   // should be width * height

// Compiles fine, but prints the wrong text:
cout << "Hello, Wrold!" << endl;  // typo in the string

Compile-time errors are the easier category to deal with: the compiler tells you exactly which line is broken. Run-time errors require you to trace the logic yourself (or use a debugger). The goal of good C++ style is to push as many errors as possible into the compile-time category, where the toolchain catches them for you.

Common first mistakes

Key rules to remember