[libcxx] r276238 - Implement std::string_view as described in http://wg21.link/P0254R1. Reviewed as https://reviews.llvm.org/D21459

[Eric Fiselier via cfe-commits]

On Thu, Jun 15, 2017 at 8:38 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

> I just started working on a patch to add #if guards, and the first
> interesting thing I found was the basic_string constructor:
>
> template <class _CharT, class _Traits, class _Allocator>
> template <class _Tp>
> basic_string<_CharT, _Traits, _Allocator>::basic_string(
>              const _Tp& __t, size_type __pos, size_type __n, const
> allocator_type& __a,
> typename enable_if<__can_be_converted_to_string_view<_CharT, _Traits,
> _Tp>::value, void>::type *)
>     : __r_(__second_tag(), __a)
> {
> __self_view __sv = __self_view(__t).substr(__pos, __n);
>     __init(__sv.data(), __sv.size());
> #if _LIBCPP_DEBUG_LEVEL >= 2
>     __get_db()->__insert_c(this);
> #endif
> }
>
>
>
That constructor was added in C++17, so removing it along with string_view
should be OK.
Assuming we don't use it to implement older constructors using a single
template.



> I suppose the decision was made so that std::string could take advantage
> of it.
>
> Is it a conforming extension?
>

No, because it can change the meaning of otherwise well defined code, as
you pointed out initially.


>
> On Jun 15, 2017, at 18:35, Eric Fiselier <eric at efcs.ca> wrote:
>
> It *shouldn't* include <string_view>, that's a given.
>
> IIRC, and Marshall would know better, I believe it was untenable to
> maintain a version of <string> that didn't depend on <string_view> after
> making
> the changes required for C++17.
>
> However inspecting <string> now it does seem possible that the entanglement
> is avoidable.Though it's also likely I'm just not seeing the whole
> picture.
>
> /Eric
>
> On Thu, Jun 15, 2017 at 6:42 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
>
>>
>> > On Jul 20, 2016, at 22:31, Marshall Clow via cfe-commits <
>> cfe-commits at lists.llvm.org> wrote:
>> >
>> > Modified: libcxx/trunk/include/string
>> > URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/str
>> ing?rev=276238&r1=276237&r2=276238&view=diff
>> > ============================================================
>> ==================
>> >
>> > @@ -435,6 +461,7 @@ basic_string<char32_t> operator "" s( co
>> > */
>> >
>> > #include <__config>
>> > +#include <string_view>
>>
>> This breaks the following, valid, C++14 code:
>>
>>     #include <string>
>>     #include <experimental/string_view>
>>     using namespace std;
>>     using std::experimental::string_view;
>>     void f() { string_view sv; }
>>
>> Should <string> #include <string_view> even when we're not in C++17
>> mode?  Why?
>>
>> > #include <iosfwd>
>> > #include <cstring>
>> > #include <cstdio>  // For EOF.
>>
>>
>
>
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