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

[Duncan P. N. Exon Smith via cfe-commits]

Ah, also the enable_if for same:

>     template<class _Tp>
>         _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
>         basic_string(const _Tp& __t, size_type __pos, size_type __n,
>                      const allocator_type& __a = allocator_type(),
>                      typename enable_if<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value, void>::type* = 0);

(Even if it is a conforming extension, it's fairly user hostile.  Doesn't mean it's necessarily wrong, but...)

> On Jun 15, 2017, at 19:38, Duncan P. N. Exon Smith via cfe-commits <cfe-commits at lists.llvm.org> 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
>> }
> 
> I suppose the decision was made so that std::string could take advantage of it.
> 
> Is it a conforming extension?
> 
>> On Jun 15, 2017, at 18:35, Eric Fiselier <eric at efcs.ca <mailto: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 <mailto:dexonsmith at apple.com>> wrote:
>> 
>> > On Jul 20, 2016, at 22:31, Marshall Clow via cfe-commits <cfe-commits at lists.llvm.org <mailto:cfe-commits at lists.llvm.org>> wrote:
>> >
>> > Modified: libcxx/trunk/include/string
>> > URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/string?rev=276238&r1=276237&r2=276238&view=diff <http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/string?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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