Single-Line Comments

The simplest way to write a comment in C++ is with two forward slashes: //. Everything on the same line after // is completely ignored by the compiler.

#include <iostream>

int main() {
    // Prints a greeting
    std::cout << "Hello!" << std::endl;
    return 0;
}

A single-line comment doesn't have to sit on its own line. It can also appear directly after code on the same line — this is called an "inline comment".

std::cout << "Hello!" << std::endl; // prints "Hello!" to the console

A single-line comment in C++ begins with the characters ____.

With //, you can also temporarily "turn off" an entire line of code without deleting it. This is called "commenting out". The compiler then treats that line as if it weren't there at all.

int main() {
    std::cout << "Line 1" << std::endl;
    // std::cout << "Line 2" << std::endl;
    std::cout << "Line 3" << std::endl;
    return 0;
}

What does this program print?

std::cout << "A" << std::endl;
// std::cout << "B" << std::endl;
std::cout << "C" << std::endl;

Important: // only comments out text up to the end of the current line. A line break automatically ends the comment — the next line is read as normal code again.

A // comment automatically also applies to the next line of code.