Your First Program

Now that you know the machine and the pipeline, let's slow down on a program you have already seen and examine every single piece. "Hello, World!" looks trivial, but the four-line version below uses roughly a dozen distinct language features. Let's name them.

Line 1 — #include <iostream>

A preprocessor directive (we met these in the last chapter). It tells the preprocessor to paste the entire contents of the standard header <iostream> here, before the compiler sees the file. <iostream> is the standard library header for stream-based input/output. It is what makes std::cout exist.

If you remove this line, the program fails with a compiler error like "std::cout was not declared in this scope."

Line 3 — int main()

The entry point of every standalone C++ program. When the operating system launches your executable, it ultimately calls a function literally named main. C++ requires this function to return an int. The return value is the program's exit status: by convention, 0 means success and any other value means a particular kind of failure.

You may also see int main(int argc, char* argv[]), which is how you receive command-line arguments. We'll meet that later.

The { ... } block

Curly braces define a compound statement, also called a scope or block. Variables declared inside a block exist only until the matching closing brace. We will use scope rules constantly when we discuss lifetimes and resource management.

Line 4 — std::cout << "Hello, World!\n";

A single C++ statement that does a lot.

• std:: is a namespace qualifier. The C++ standard library lives in the namespace std, so its names are spelled std::cout, std::string, std::vector, and so on. Namespaces prevent collisions between libraries that happen to pick the same short names.
• cout is the name of the standard output stream object — a pre-existing object that represents the place where text goes when you print it.
• << is the stream insertion operator. Despite the symbol it is not really "bit-shift left" here; it is a function call in disguise on cout that means "send the right-hand thing into this stream."
• "Hello, World!\n" is a string literal: a sequence of bytes the compiler embedded directly into the data segment of your program. The trailing \n is the escape sequence for a newline character.
• ; ends the statement. C++ statements end with a semicolon, the same way English sentences end with a period.

Line 5 — return 0;

Hand control back to the operating system with exit status 0. If you return 1; instead, scripts that called your program (like a shell, a CI pipeline, or a parent process) will see "this program failed." The numerical exit code is one of the simplest ways programs communicate with each other.

In a real shell you could check the exit code with echo $?. The browser sandbox does not show it, but the OS would.

A slightly bigger first program

Below is a more interesting first program. It uses variables, input, a function, and a loop. Don't worry if some pieces feel new; we'll spend whole chapters on each.

A quick tour of the new pieces:

• #include <string> brings in std::string, a managed, heap-backed sequence of characters that handles its own memory.
• std::string shout(const std::string& s) declares a function named shout that takes a std::string by const reference (we'll explain references later) and returns a std::string.
• for (char c : s) is a range-based for loop: for each character c in the string s, do the body.
• if / else is C++'s conditional statement.
• out += ... appends to the output string. C++ strings know how to grow themselves.
• The main loop runs shout three times and prints the result.

Edit the greeting, change the loop count, or change the test inside if to convert other character ranges. Then run it.

Comments

Comments are notes you leave for yourself or future readers. The compiler ignores them.

// This is a single-line comment.

/*
   This is a multi-line
   comment.
*/

Use comments to explain why code does something, not what it does. The code already shows the what; a good comment captures intent or constraints.

Common beginner mistakes

A first challenge

Time to write something yourself. Don't peek at the solution unless you really need to.

Ready
Set
Go!

Test your understanding

Up next: how to think about programming problems before writing any code at all.