[cfe-dev] literal suffixes for std::basic_string?

[Marshall Clow]

On Jul 22, 2013, at 11:59 AM, Eli Friedman <eli.friedman at gmail.com> wrote:

> On Mon, Jul 22, 2013 at 11:22 AM, Marshall Clow <mclow.lists at gmail.com> wrote:
>> I'm trying to implement N3642 in libc++ (User-defined literals for standard library types) and I'm running into something I don't understand.
>> 
>> I tried to implement them in a standalone program, and got errors from clang:
>>> string_literal.cpp:6:21: warning: user-defined literal suffixes not starting
>>>      with '_' are reserved; no literal will invoke this operator
>> 
>> Ok, fine. I developed my (and tested my code) using underscores. That all works (see attached "string_literal.cpp")
>> 
>> [ All the code here was built with TOT clang and "-std=c++1y -stdlib=libc++ -I $LLVM/libcxx/include"  ]
>> 
>> Then I dropped the code into libc++'s <string> header and removed the underscores (patch attached).
>> No warnings - so apparently clang can tell when the code is in a system header.
> 
> Yes... but it's probably just suppressing the warning, not actually
> changing the behavior.
> 
>> But if I uncomment the code that uses those operators (starting about line 46 of string_literal.cpp), I get the following errors:
>> 
>>> string_literal.cpp:46:48: error: invalid suffix on literal; C++11 requires a
>>>      space between literal and identifier [-Wreserved-user-defined-literal]
>>>        static_assert ( std::is_same<decltype(   "hi"s), std::string>::v...
>>>                                                     ^
>>> 
>>> string_literal.cpp:46:48: error: expected ')'
>>> string_literal.cpp:46:40: note: to match this '('
>>>        static_assert ( std::is_same<decltype(   "hi"s), std::string>::v...
>>>                                             ^
>>> string_literal.cpp:46:3: error: static_assert failed ""
>>>        static_assert ( std::is_same<decltype(   "hi"s), std::string>::value, "...
>>>        ^               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> 
>> which, it seems to me, are just wrong.
>> 
>> Maybe I screwed up putting the stuff into <string> (but that looks ok to me).
>> I thought I might be using the wrong set of libc++ header files, but I checked, and I don't think that's happening, either.
>> 
>> Any suggestions?
> 
> User-defined literals without underscores are reserved... and so
> without any guidance from the standard as to what they are supposed to
> do, clang just ignores them outright.

Um, I don't think that's right.

They are reserved for standard library implementations, and the C++14 draft defines several such suffixes that library implementers must implement.

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

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