Design question LWG 2861: basic_string should require that charT match traits::char_type

Jonathan Wakely jwakely.gcc@gmail.com
Mon Mar 13 17:31:00 GMT 2017


On 13 March 2017 at 11:33, Daniel Krügler <daniel.kruegler@gmail.com> wrote:
> 2017-03-13 11:56 GMT+01:00 Jonathan Wakely <jwakely.gcc@gmail.com>:
>> On 12 March 2017 at 13:21, Daniel Krügler <daniel.kruegler@gmail.com> wrote:
>>> I'm now working on
>>>
>>> http://cplusplus.github.io/LWG/lwg-defects.html#2861
>>>
>>> The new wording state is now equivalent to basic_string_view, whose
>>> current implementation doesn't bother verifying the requirement, so
>>> this code (which as UB) currently compiles just fine:
>>>
>>> #include <string>
>>> #include <string_view>
>>>
>>> struct MyTraits : std::char_traits<char>
>>> {
>>>   typedef unsigned char char_type;
>>> };
>>>
>>> int main()
>>> {
>>>   std::basic_string<char, MyTraits> my_string;
>>>   std::basic_string_view<char, MyTraits> my_string_view;
>>> }
>>>
>>> So the least I could do is just - nothing. But it seems to me that we
>>> could protect users from doing such silly things by adding a
>>> static_assert to both basic_string and basic_string_view, the former
>>> being equivalent to
>>>
>>> #if __cplusplus >= 201103L
>>>       static_assert(__are_same<value_type, _CharT>::value,
>>>                     "traits_type::char_type must be equal to _CharT");
>>> #endif
>>>
>>> and the latter an unconditional
>>>
>>>       static_assert(is_same<typename _Traits::char_type, _CharT>::value,
>>>                     "traits_type::char_type must be equal to _CharT");
>>>
>>> Would you agree with that course of action?
>>
>> Not at this stage of gcc7 development. If the silly code compile fine
>> then we risk breaking working code, and we're too close to a release
>> to do that.
>
> Is there a way to mark a patch suggestion for gcc8 and is so, how?

Just mention it in the email. Ideally ping the patch after gcc7 is
released so someone (probably me) can apply it once we're back in
Stage 1.



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