> On 2017-Jun-16, at 05:58, Duncan Exon Smith <dexonsm...@apple.com> wrote: > > > On Jun 15, 2017, at 22:22, Eric Fiselier <e...@efcs.ca <mailto:e...@efcs.ca>> > wrote: > >> >> >> On Thu, Jun 15, 2017 at 11:00 PM, Duncan P. N. Exon Smith >> <dexonsm...@apple.com <mailto:dexonsm...@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 <e...@efcs.ca >>> <mailto:e...@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 >>> <dexonsm...@apple.com <mailto:dexonsm...@apple.com>> wrote: >>> >>>> On Jun 15, 2017, at 19:42, Eric Fiselier <e...@efcs.ca >>>> <mailto:e...@efcs.ca>> wrote: >>>> >>>> >>>> >>>> On Thu, Jun 15, 2017 at 8:38 PM, Duncan P. N. Exon Smith >>>> <dexonsm...@apple.com <mailto:dexonsm...@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 <e...@efcs.ca >>>>> <mailto:e...@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 >>>>> <dexonsm...@apple.com <mailto:dexonsm...@apple.com>>wrote: >>>>> >>>>> > On Jul 20, 2016, at 22:31, Marshall Clow via cfe-commits >>>>> > <cfe-commits@lists.llvm.org <mailto:cfe-commits@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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