< cpp

Comments serve as a sort of in-code documentation. When inserted into a program, they are effectively ignored by the compiler; they are solely intended to be used as notes by the humans that read source code. Although specific documentation is not part of the C++ standard, several utilities exist that parse comments with different documentation formats.


/* comment */ (1)
// comment (2)
1) Often known as "C-style" or "multi-line" comments.
2) Often known as "C++-style" or "single-line" comments.

All comments are removed from the program at translation phase 3 by replacing each comment with a single whitespace character.


C-style comments are usually used to comment large blocks of text, however, they can be used to comment single lines. To insert a C-style comment, simply surround text with /* and */; this will cause the contents of the comment to be ignored by the compiler. Although it is not part of the C++ standard, /** and */ are often used to indicate documentation blocks; this is legal because the second asterisk is simply treated as part of the comment. C-style comments cannot be nested.


C++-style comments are usually used to comment single lines, however, multiple C++-style comments can be placed together to form multi-line comments. C++-style comments tell the compiler to ignore all content between // and a new line.


Because comments are removed before the preprocessor stage, a macro cannot be used to form a comment and an unterminated C-style comment doesn't spill over from an #include'd file.

Besides commenting out, other mechanisms used for source code exclusion are

#if 0
    std::cout << "this will not be executed or even compiled\n";


if(false) {
    std::cout << "this will not be executed\n"


/* C-style comments can contain
multiple lines */
/* or just one */
// C++-style comments can comment one line
// or, they can
// be strung together
int main()
  // The below code won't be run
  // return 1;
  // The below code will be run
  return 0;

See Also