[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
Sun Jun 18 10:46:06 PDT 2017
> On 2017-Jun-16, at 05:58, Duncan Exon Smith <dexonsmith at apple.com> wrote:
>
>
> On Jun 15, 2017, at 22:22, Eric Fiselier <eric at efcs.ca <mailto:eric at efcs.ca>> wrote:
>
>>
>>
>> On Thu, Jun 15, 2017 at 11:00 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto: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
FTR, it also breaks code that relies on string_view::clear(), which disappeared.
>>> On Jun 15, 2017, at 21:55, Eric Fiselier <eric at efcs.ca <mailto: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 *.
I had a bit of a look at experimental/filesystem, and it relies pretty heavily on the string/string_view conversions. I still feel like this approach might be "the right one", but perhaps it's not worth it.
>>> /Eric
>>>
>>> On Thu, Jun 15, 2017 at 10:51 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
>>>
>>>> On Jun 15, 2017, at 19:42, Eric Fiselier <eric at efcs.ca <mailto:eric at efcs.ca>> wrote:
>>>>
>>>>
>>>>
>>>> On Thu, Jun 15, 2017 at 8:38 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto: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 <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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