[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 cfe-commits at lists.llvm.org
Thu Jun 15 19:38:35 PDT 2017


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> 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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