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.
Contents |
/*comment*/ | (1) | ||||||||
//comment | (2) | (since C99) | |||||||
All comments are removed from the program attranslation phase 3 by replacing each comment with a single whitespace character.
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 acharacter constant, astring 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++-styleC++-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 Except within acharacter constant, astring literal, or a comment, the characters // y = f(x); // invoke algorithmA 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) |
Because commentsare 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 0puts("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; */
#include <stdio.h>/*C-style comments can containmultiple 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 runputs("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:
WorldHello, again
C++ documentation forComments |
