十六进制或二进制数字中的数字之前是否允许使用数字分隔符？ [英] Are digit separators allowed before the digits in a hex or binary number?

问题描述

C ++ 14沿 3’141’592’653’589 的概念将数字分隔符的概念引入了文字。现在，这是可读代码的伟大功能，但我想知道它是否允许在 0x / 0b 之前引号。 $ c>类型的文字。在我看来：

C++14 introduced the concept of digit separators into literals, along the lines of 3'141'592'653'589. Now this is a great feature for readable code but I was wondering whether it allowed quotes before the numeric portion of a 0x/0b-type literal. It seems to me that:

unsigned int topThreeBits = 0b'1110'0000;
unsigned int hexNum       = 0x'dead'beef;

比没有前导分隔符的更具可读性：

is more readable than the one without a leading separator:

unsigned int topThreeBits = 0b1110'0000;
unsigned int hexNum       = 0xdead'beef;

因为它清楚地从数字中划出了基数。

because it clearly delineates the base from the digits.

由于我还没有C ++ 14编译器，因此我需要一种或另一种方式确认它是否允许这样做。

Since I don't yet have a C++14 compiler, I need confirmation one way or another as to whether it allows this.

我知道对于像'123'456 这样的非前缀数字没有意义，尤其是因为解析器不知道它是否应该是 char 变量或数字文字。

I know it doesn't make sense for un-prefixed numbers like '123'456, especially since the parser wouldn't know if it was meant to be a char variable or a numeric literal.

但是，对于带前缀的文字，我看不到令牌的含义有任何混淆在第一个'到达的时刻- 0x / 0b 已经规定它将是一个数字文字

But, for prefixed literals, I can't see there's any confusion as to what the token is meant to be at the point the first ' arrives - the 0x/0b has already dictated it's going to be a numeric literal.

推荐答案

如果我们从草稿C ++ 14标准：N4140 部分 2.14.2 [lex.icon] ，不允许在十六进制的基本指示符之后或二进制文字：

If we look at the grammar from the draft C++14 standard: N4140 section 2.14.2 [lex.icon], it is not allowed right after the base indicator of hexadecimal or binary literals:

binary-literal:
  0b binary-digit
  0B binary-digit
  binary-literal ’opt binary-digit
[...]
hexadecimal-literal:
  0x hexadecimal-digit
  0X hexadecimal-digit
  hexadecimal-literal ’opt hexadecimal-digit

尽管，八进制文字的确允许在基本指示符之后使用分隔符：

Although, octal literals do allow the separator after the base indicator:

octal-literal:
  0
  octal-literal ’opt octal-digit

我们还可以使用提供C ++ 14编译器的在线编译器之一进行检查，例如 Coliru 或 Wandbox 。

We can also check using one of the online compiler which provide C++14 compilers such as Coliru or Wandbox.

跟踪此更改的Evolution Working Group问题是问题27：N3781单引号标记为数字分隔符，N3661，N3499数字分隔符，N3448无痛数字分隔。我认为此设计决定没有明显的理由，也许它只是数字分隔符的字面解释。

The Evolution Working Group issue which tracked this change was issue 27: N3781 Single-Quotation-Mark as a Digit Separator, N3661, N3499 Digit Separators, N3448 Painless Digit Separation. I don't see an obvious rationale for this design decision, perhaps it is solely a literal interpretation of digit separator.

请注意，我们可以找到在哪里可以找到当前的C或C ++标准文档？。

Note we can find a list of the draft standards from Where do I find the current C or C++ standard documents?.

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