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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.

-

Syntax

- - -
/* comment */ (1)
// comment (2) (since C99)
-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

C-style comments are usually used to comment large blocks of text or small fragments of code; however, they can be used to comment single lines. To insert text as a C-style comment, simply surround the text with /* and */. C-style comments tell the compiler to ignore all content between /* and */. 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.

-

Except within a character constant, a string literal, or a comment, the characters /* introduce a comment. The contents of such a comment are examined only to identify multibyte characters and to find the characters */ that terminate the comment. C-style comments cannot be nested.

- - -

C++-style

C++-style comments are usually used to comment single lines of text or code; however, they can be placed together to form multi-line comments. To insert text as a C++-style comment, simply precede the text with // and follow the text with the new line character. C++-style comments tell the compiler to ignore all content between // and a new line.

-

Except within a character constant, a string literal, or a comment, the characters // introduce a comment that includes all multibyte characters up to, but not including, the next new-line character. The contents of such a comment are examined only to identify multibyte characters and to find the new-line character that terminates the comment. C++-style comments can be nested:

-
//  y = f(x);   // invoke algorithm

A C-style comment may appear within a C++-style comment:

-
//  y = f(x);   /* invoke algorithm */

A C++-style comment may appear within a C-style comment; this is a mechanism for excluding a small block of source code:

-
/*
-    y = f(x);   // invoke algorithms
-    z = g(x);
-*/
 (since C99)

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.

-
/* An attempt to use a macro to form a comment. */
-/* But, a space replaces characters "//".       */
-#ifndef DEBUG
-    #define PRINTF //
-#else
-    #define PRINTF printf
-#endif
-...  
-PRINTF("Error in file %s at line %i\n", __FILE__, __LINE__);

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

-
#if 0
-    puts("this will not be compiled");
-    /* no conflict with C-style comments */
-    // no conflict with C++-style comments
-#endif

and

-
if(0) {
-    puts("this will be compiled but not be executed");
-    /* no conflict with C-style comments */
-    // no conflict with C++-style comments
-}

The introduction of // comments in C99 was a breaking change in some rare circumstances:

-
a = b //*divisor:*/ c
-+ d; /* C89 compiles a = b / c + d;
-        C99 compiles a = b + d; */

Example

#include <stdio.h>
-/*
-C-style comments can contain
-multiple lines.
-*/
- 
-/* Or, just one line. */
- 
-// C++-style comments can comment one line.
- 
-// Or, they can
-// be strung together.
- 
-int main(void)
-{
-  // The below code won't be run
-  // puts("Hello");
- 
-  // The below code will be run
-  puts("World");
- 
-  // A note regarding backslash + newline.
-  // Despite belonging to translation phase 2 (vs phase 3 for comments),
-  // '\' still determines which portion of the source code is considered
-  // as 'comments':
-  // This comment will be promoted to the next line \
-  puts("Won't be run"); // may issue a warning "multi-line comment"
-  puts("Hello, again");
-}

Output:

-
World
-Hello, again

References

See also

-
C++ documentation for Comments
-

- © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
- https://en.cppreference.com/w/c/comment -

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