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

[Duncan Exon Smith via cfe-commits]

> On Jun 15, 2017, at 22:22, Eric Fiselier <eric at efcs.ca> wrote:
> 
> 
> 
>> On Thu, Jun 15, 2017 at 11:00 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>> Your suggestion is essentially to replace experimental/string_view with something like:
>> 
>>     namespace std { inline namespace __1 { namespace experimental {
>>       template <class CharT>
>>       using basic_string_view = _VSTD::basic_string_view;
>>     }}}
>> 
>> That breaks:
>> 1. User compiles 1.cpp with older toolchain.  1.cpp implements foo(std::experimental::string_view).
>> 2. User compiles 2.cpp with newer toolchain.  2.cpp calls foo(std::experimental::string_view).
>> 3. User links 1.o with 2.o.
>> 
>> I'm not sure if this matters.
> 
> It can't matter. <experimental/foo> are allowed to break both their API and ABI as needed.
> 
> Also I was suggesting 
> 
>    namespace std { namespace experimental {
>      using std::basic_string_view;
>      using std::string_view;
>   }}
>  
> This approach will break code that expects experimental::string_view and std::string_view are different types:
> Example:
> 
>   void foo(std::string_view);
>   void foo(std::experimental::string_view);
>   foo(std::string_view{}); // ambiguous
> 
>> 
>>> On Jun 15, 2017, at 21:55, Eric Fiselier <eric at efcs.ca> wrote:
>>> 
>>> I would also want to do serious performance analysis on this patch. Does removing the string_view overloads cause less optimal overloads to be chosen? Perhaps allocating ones?
>>> That would be really unfortunate, and I'm not sure that's in the best interest of our users at large.
>> 
>> Less optimal compared to what?  C++17 code?
> 
> Not sure yet, I'm trying to figure out what types the `const Tp&` overloads
> are attempting to soak up. Is it only string_view? 

The type trait restricts it to things convertible to string_view that are not const char *.

> 
>  
>> 
>>> /Eric
>>> 
>>>> On Thu, Jun 15, 2017 at 10:51 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>>>> 
>>>>> On Jun 15, 2017, at 19:42, Eric Fiselier <eric at efcs.ca> wrote:
>>>>> 
>>>>> 
>>>>> 
>>>>> 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. 
>>>> 
>>>> Let me know if this patch is along the right lines.  If so, I'll finish it up and put it on phab.
>>>> 
>>>> experimental/filesystem/path.cpp doesn't compile, since experimental/filesystem uses things like operator+=(string, string_view) extensively.  But I'd like an early opinion on the approach before I dig in.
>>>> 
>>>> In string, the only function that needed to be rewritten was string::compare(size, size, string, size, size).  I'm nervous that filesystem will be a bigger job.
>>>> 
>>>> 
>>>> 
>>>>>  
>>>>>> 
>>>>>>> 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/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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