Your First C++ Program

Time to understand every line of the program you saw:

#include <iostream>
using namespace std;

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

Line 1: #include <iostream>

C++ ships with pre-written code organized into files called header files. iostream (short for input/output stream) is the header that contains the printing and keyboard-reading tools. #include copies those tools into your program.

Without this line, the compiler has never heard of cout and refuses to compile.

Line 2: using namespace std;

All the standard C++ tools live in a named group (a namespace) called std, short for standard. This line says: "when I write cout, I mean the one from std." Without it, you'd have to write std::cout every time. Both styles are valid — this course uses the short one for readability.

Line 4: int main() { ... } main is where every C++ program starts running — the entry point. When you run your program, the computer finds main and executes the instructions between its braces { }, top to bottom.

Every program must have exactly one main. (You're looking at your first function — a named block of code. Module 5 is entirely about writing your own.)

Line 5: cout << "Hello, World!" << endl; cout (console output, pronounced "see-out") is how C++ prints to the screen. The << arrows push whatever is on their right into cout:

cout << "Hello";        // prints text
cout << 42;              // prints a number
cout << "Age: " << 30;  // chain several things: Age: 30

endl (end line) moves to the next line, like pressing Enter. Line 6: return 0;

When main finishes, it reports a number to the operating system. By universal convention, 0 means "everything went fine" and any other number means "something went wrong".

Two rules of C++ grammar you must know from day one.

Rule 1: statements end with a semicolon.

A statement is one complete instruction — like one sentence. In C++, the 'period' at the end of the sentence is the semicolon ;:

cout << "Hi";      // one statement
return 0;           // another statement

Forget a semicolon and the compiler sees two sentences running together — it stops with an error.

Rule 2: braces group code, indentation is decoration.

The braces { } after main() mark where the program's instructions begin and end. The spaces at the start of each line (indentation) are purely for human eyes — the compiler ignores them completely. We indent anyway, always, because readable code matters as much as working code.