Comments
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.
Syntax
/* multi-line-content */
|
(1) | ||||||||
// single-line-content
|
(2) | ||||||||
| multi-line-content | - | A character sequence which does not contain the character sequence */
|
| single-line-content | - | A character sequence which does not contain any new line character |
All comments are removed from the program at translation phase 3 by replacing each comment with a single whitespace character.
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.
C-style comments cannot be nested:
/************************
/* Comment ends here -> */
************************/ // <- Not part of comment
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.
Notes
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.
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";
}
Example
#include <iostream>
/* 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;
}
Output:
1 hello world
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| P2843R3 | C++98 | if single-line-content contains a form-feed or a vertical-tab character, only whitespace characters are allowed till the end |
remove the restriction |
See also
C documentation for comment
|
