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;
is more readable than the one without a leading separator:
unsigned int topThreeBits = 0b1110'0000;
unsigned int hexNum = 0xdead'beef;
because it clearly delineates the base from the digits.
Since I don't yet have a C++14 compiler, I need confirmation one way or another as to whether it allows this.
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.
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.
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
Although, octal literals do allow the separator after the base indicator:
octal-literal:
0
octal-literal ’opt octal-digit
We can also check using one of the online compiler which provide C++14 compilers such as Coliru or Wandbox.
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.
Note we can find a list of the draft standards from Where do I find the current C or C++ standard documents?.