Comments - cppreference.com (original) (raw)

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.

[edit] Syntax

| | | | | ----------------------- | --- | | | /* comment */ | (1) | | | | | | | // comment | (2) | | | | | |

Often known as "C-style" or "multi-line" comments.
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.

[edit] C-style

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.

[edit] C++-style

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.

[edit] Notes

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"; #endif

and

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

[edit] Example

#include /* C-style comments can contain multiple lines / / or just one / /*************

you can insert any *, but
you can't make comments nested */

// C++-style comments can comment one line // or, they can // be strung together int main() { // comments are removed before preprocessing, // so ABC is "1", not "1//2134", and "1 hello world" // will be printed #define ABC 1//2134 std::cout << ABC << " hello world\n"; // The below code won't be run // return 1; // The below code will be run return 0; }