c++ - 为什么成对注释不能放在 C++ 中的字符串中？

Question

通常里面的任何东西/*和 */被视为评论。

但是声明， std::cout << "not-a-comment /* comment */"; 版画 not-a-comment /* comment */而不是 not-a-comment .

为什么会这样？在 C++ 中还有其他地方我不能使用注释吗？

Answer 1

这是最大咀嚼原则的结果。这是 C++ 语言遵循的词法规则。处理源文件时，翻译分为(逻辑)阶段。在第 3 阶段，我们得到预处理 token :

[lex.phases]

1.3 The source file is decomposed into preprocessing tokens and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or in a partial comment. Each comment is replaced by one space character. New-line characters are retained.

将注释转换为空白 pp-token 是在同一阶段完成的。现在字符串文字是一个 pp-token:

[lex.pptoken]

preprocessing-token:
  header-name
  identifier
  pp-number
  character-literal
  user-defined-character-literal
  string-literal
  user-defined-string-literal
  preprocessing-op-or-punc
  each non-white-space character that cannot be one of the above

与其他文字一样。最大咀嚼原理告诉我们:

3 If the input stream has been parsed into preprocessing tokens up to a given character:

• Otherwise, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token, even if that would cause further lexical analysis to fail, except that a header-name is only formed within a #include directive.

因此，因为预处理找到了开头的 "，它必须继续寻找最长的字符序列，这些字符将构成有效的 pp-token(在这种情况下，token 是一个字符串文字)。这序列在结束 " 处结束。这就是它不能停止和处理评论的原因，因为它有义务消耗到结束的引号。

按照这些规则，您可以查明评论不会被预处理器作为评论处理的地方。