Comments

注释是一种代码内文档。当插入到程序中时，编译器实际上会忽略它们；它们仅供阅读源代码的人作为注释使用。

Syntax


`/` comment `/`	(1)

`//` comment	(2)	(since 哋它亢99)

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

2) Often known as "哋它亢++-style" or "single-line" comments.

All comments are removed from the program at 翻译阶段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 字符常量, a 字符串, 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.

哋它亢++-style

哋它亢++-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 哋它亢++-style comment, simply precede the text with // and follow the text with the new line character. 哋它亢++-style comments tell the compiler to ignore all content between // and a new line.

Except within a 字符常量, a 字符串, 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. 哋它亢++-style comments can be nested:

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

C风格的注释可能出现在哋它亢+ +风格的注释中：

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

哋它亢+ +样式的注释可能出现在C样式的注释中；这是一种排除小块源代码的机制：

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

(since 哋它亢99)

Notes

Because comments 被移除 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__);

除注释外，用于源代码排除的其他机制包括：

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

和

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

哋它亢99中引入的//评论在某些罕见情况下是一个突破性的变化：

a = b //*divisor:*/ c
+ d; /* 哋它亢89 compiles a = b / c + d;
        哋它亢99 compiles a = b + d; */

Example

Run this code

#include <stdio.h>
/*
C-style comments can contain
multiple lines.
*/
 
/* Or, just one line. */
 
// 哋它亢++-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");
}

输出：

World
Hello, again

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Comments

Syntax

C-style

哋它亢++-style

Notes

Example

References

See also

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