Basic Structure of a C++ Program¶

Every C++ program is built from the same handful of pieces. This chapter walks through them using the smallest program that does something useful.

#include <iostream>

int main() {
    std::cout << "Hello, world!" << "\n";
    return 0;
}

▶ Run on Compiler Explorer

`#include`: pulling in code¶

Lines that start with # are preprocessor directives. They are handled before the real compilation begins.

#include <iostream> tells the compiler: "before you read the rest of this file, paste in the contents of the iostream header." That header is what defines std::cout and friends. Without it, the compiler will not know what std::cout means and your code will not compile.

Angle brackets (<iostream>) are used for the standard library and system headers. Quotes ("my_header.hpp") are used for your own files. We will see this distinction in Chapter 2.

`main`: where execution starts¶

int main() {
    // ...
    return 0;
}

Every C++ program has exactly one function called main. The operating system calls it to start your program. The int in front declares that main returns an integer: 0 for success, anything non-zero to signal an error.

The body of main, between { and }, is the code that actually runs.

Escape sequences¶

Look again at the first program: it prints "Hello, world!" followed by "\n". That \n is not the two characters backslash and n. Inside a string, a backslash starts an escape sequence — a stand-in for a character you cannot easily type between quotes. \n means "start a new line."

The handful you will meet constantly:

Escape	Means
`\n`	new line
`\t`	tab
`\"`	a double quote (without ending the string)
`\\`	a single backslash

The last two exist because " and \ already have special jobs — one ends the string, the other starts an escape — so you spell the literal character by escaping it. This is also why a Windows path like "C:\dev" misbehaves: \d looks like an escape. (More on that in Computer Basics.)

Statements and semicolons¶

A statement is one instruction. In C++, every simple statement — a declaration, an assignment, a function call — ends with a semicolon:

int quantity = 10;
double price = 5.40;
double sum = price * quantity;
std::cout << "Total: " << sum << "\n";

A block ({ ... }) and a control statement (an if, a for, a while) do not take a trailing semicolon — the closing brace ends them. You will meet blocks and control statements in the pages ahead.

Forgetting a semicolon is the single most common error a beginner gets. The compiler error usually points to the line after the missing semicolon, which is confusing the first time. Always check the line above too.

Blocks and scope¶

A block is code wrapped in curly braces { ... }. Blocks group statements together and define scope: the region of code in which a variable exists.

#include <iostream>

int main() {
    int x = 5;

    {
        int y = 10;
        std::cout << x << " " << y << "\n"; // both visible
    }

    // y no longer exists here
    std::cout << x << "\n"; // x still exists
}

▶ Run on Compiler Explorer

Two rules cover almost every case you will meet:

A variable declared in a block is destroyed when execution leaves that block.
An inner block can see variables from the outer block, but not the reverse.

If an inner block declares a variable with the same name as one outside, the inner one shadows the outer one: inside the inner block, the name refers to the new variable. Shadowing is legal but rarely what you want; pick distinct names.

Comments¶

Comments are notes for human readers; the compiler ignores them. C++ has two forms — // runs to the end of the line, and /* … */ can span several:

int retries = 5;          // the sensor often drops its first read, so retry a few times

/* The handshake must finish within one second, or the
   controller treats the device as missing and moves on. */
int timeoutMs = 1000;

Notice what those comments do: they explain why the values are 5 and 1000 — something the numbers alone cannot tell a future reader. A good comment explains why, not what. The code already says what it does; one that merely restates it — int retries = 5; // set retries to 5 — adds nothing. Beginner code collects that kind; resist it, and spend comments on the reasoning, the trade-off, or the surprise.

Putting it together¶

A complete program that reads two numbers from the user and prints their sum:

#include <iostream>

int main() {
    int a = 0;
    int b = 0;

    std::cout << "Enter two integers separated by a space: ";
    std::cin >> a >> b;

    int sum = a + b;
    std::cout << "Sum: " << sum << "\n";

    return 0;
}

std::cin is the input counterpart of std::cout: where << sends values out to the screen, >> reads a value in from the keyboard and stores it in a variable. Here std::cin >> a >> b reads two integers.

You now know every structural element this program is built from: #include, main, blocks, statements, semicolons, comments. The remaining chapters of this section fill in what goes inside main: variables, operators, control flow, and functions.

A note on style¶

Two stylistic conventions worth adopting from day one:

Indent the body of every block by four spaces. Modern IDEs do this automatically. Keep it consistent.
Always use braces, even for one-line if and while bodies. It is one line of extra typing and removes a whole class of bugs:

// Avoid:
if (ready) doThing();

// Prefer:
if (ready) {
    doThing();
}

The second form is harder to break when someone adds a second line later.