C++ Comments

A comment is a note in code that a person writes for other readers. The compiler ignores comments when it builds a program.

A line comment

The comment sits on its own line. The print line below it still runs.

#include <iostream>

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

A block comment around a statement

The block comment covers one whole statement. That statement does not run.

#include <iostream>

int main() {
  /* std::cout << "Hidden" << std::endl; */
  std::cout << "Visible" << std::endl;
  return 0;
}

A trailing comment

The comment follows code on the same line. The code still runs as usual.

#include <iostream>

int main() {
  std::cout << "Done" << std::endl; // Mark the end
  return 0;
}

Example

#include <iostream>

int main() {
  // Print the plan for today
  std::cout << "Plan for today" << std::endl;
  std::cout << "Write one program" << std::endl; // The second line of the plan
  return 0;
}

Key points

Use // to start a line comment.
A line comment runs to the end of its line.
Use /* to start a block comment and */ to end it.
A block comment can hide a statement so it does not run.
A trailing comment sits after code on the same line.

In this example, a line comment sits above the first print line. The first line prints Plan for today. A trailing comment sits after the second print line, and that line prints Write one program.

Note

A comment is for readers, not for the computer.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Programiz C++ Compiler

#include <iostream>

int main() {
  // Print the plan for today
  std::cout << "Plan for today" << std::endl;
  std::cout << "Write one program" << std::endl; // The second line of the plan
  return 0;
}

Exercise

Which marks the start of a line comment?

// /* % #