Are C++14 digit separators allowed in user defined literals?

问题

While clang compiles the following line, g++ 6.1 complains about the digit separator (see live example on Coliru):

auto time = 01'23s;

Which compiler, if any, is correct according to the C++14 standard (N3796)?

Otherwise, is allowing digit separators (§2.14.2) just an implementation detail in the user-defined literals (§2.14.8) of the <chrono> library (§20.12.5.8)? IMHO it should be not, since these literals are defined on unsigned long long parameters.

I remember Howard Hinnant using 10'000s as an example during his CppCon 2016 talk "A <chrono> tutorial" (at about 42 minutes in his talk).

(Please note, that I did not intend to code "1 minute and 23 seconds", which is only correct by accident, since the octal literal 0123 is 64 + 16 + 3 == 83. For that purpose I should write

auto time = 1min + 23s;

but that possible misleading interpretation is not part of the question.)

回答1:

If you look at the grammar, user-defined-integer-literal can be octal-literal ud-suffix, and octal-literal is defined as either 0 or octal-literal ’_opt octal-digit.

N4140 §2.14.8

user-defined-literal:

user-defined-integer-literal

[...]

user-defined-integer-literal:

octal-literal ud-suffix

[...]

N4140 §2.14.2

octal-literal:

0

octal-literal ’_opt octal-digit

So 01'23s is a perfectly valid literal.

回答2:

WLOG for decimal literals:

[lex.ext]:

user-defined-integer-literal:
decimal-literal ud-suffix

[lex.icon]:

decimal-literal:
nonzero-digit
decimal-literal ’_opt digit

I.e. yes, digit separators are allowed in UDLs.

回答3:

This seems to be a bug in GCC's implementation of the <chrono> library, as @Aaron McDaid suggested. There is a (currently unconfirmed) bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69905

GCC's libstdc++ implements two signatures for std::chrono_literals:

constexpr chrono::duration<long double>
operator""s(long double __secs)
{ return chrono::duration<long double>{__secs}; }

template <char... _Digits>
  constexpr chrono::seconds
  operator""s()
  { return __check_overflow<chrono::seconds, _Digits...>(); }

The template version giving the error is not required by the standard. When adding

constexpr chrono::seconds
operator""s(unsigned long long __secs)
{ return chrono::seconds{__secs}; }

to the <chrono> header (of my local installation) the error dissappears.

However, GCC's library implementers may have left out this version on purpose, so they can prevent an unwanted unsigned to signed conversion, since seconds are defined as

typedef duration<int64_t> seconds;

Edit:

As recently pointed out by Jonathan Wakely in the comments of the bug report, the implementation was chosen by design in connection with an open Library Working Group issue, but did not take digit separators into account.

来源：https://stackoverflow.com/questions/40262231/are-c14-digit-separators-allowed-in-user-defined-literals

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c++

c++14

language-lawyer

user-defined-literals

digit-separator