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On 04/09/17 16:48 +0100, Jonathan Wakely wrote:
On 30/07/17 15:01 +0200, Daniel Krügler wrote:2017-07-28 22:40 GMT+02:00 Daniel Krügler <daniel.kruegler@gmail.com>:2017-07-28 22:29 GMT+02:00 Daniel Krügler <daniel.kruegler@gmail.com>:2017-07-28 22:25 GMT+02:00 Tim Song <t.canens.cpp@gmail.com>:On Fri, Jul 28, 2017 at 4:10 PM, Daniel Krügler <daniel.kruegler@gmail.com> wrote:+ // Performs an implicit conversion from _Tp to __sv_type. + template<typename _Tp> + static __sv_type _S_to_string_view(const _Tp& __svt) + { + return __svt; + }I might have gone for + static __sv_type _S_to_string_view(__sv_type __svt) noexcept + { + return __svt; + } With that, we can also use noexcept(_S_to_string_view(__t)) to make up for the absence of is_nothrow_convertible (basically the same thing I did in LWG 2993's PR).Agreed, that makes very much sense. I will adjust the P/R, but before I resubmit I would like to get feedback whether the other two compare functions also should become conditionally noexcept.Locally I have now performed the sole change of the _S_to_string_view declaration getting rid of the template, but would also like to gather feedback from the maintainers whether I should also change the form of the conditional noexcept to use the expression noexcept(_S_to_string_view(__t)) instead of the current is_same<_Tp, __sv_type>::value as suggested by Tim Song. I'm asking also, because I have a paper proposing to standardize is_nothrow_convertible submitted for the upcoming C++ mailing - This would be one of the first applications in the library ;-)A slightly revised patch update: It replaces the _S_to_string_view template by a simpler _S_to_string_view function as of Tim Song's suggestion, but still uses the simplified noexcept specification deferring it to a future application case for is_nothrow_convertible. Furthermore now all three compare function templates are now (conditionally) noexcept by an (off-list) suggestion from Jonathan Wakely.I've committed this, after some whitespace fixes and testing. Thanks!
This change causes two regressions in C++17 mode, see https://gcc.gnu.org/ml/gcc-testresults/2017-09/msg01674.html FAIL: 21_strings/basic_string/cons/char/moveable2.cc execution test FAIL: 21_strings/basic_string/cons/wchar_t/moveable2.cc execution test Here's a reduced version of that test, which passes in C++14 and fails in C++17: #include <string> #include <assert.h> class tstring : public std::string { public: tstring() : std::string() {} tstring(tstring&& s) : std::string(std::move(s)) {} }; int main() { tstring b; b.push_back('1'); tstring c(std::move(b)); assert( c.size() == 1 && c[0] == '1' ); assert( b.size() == 0 ); } The second assertion fails, because this mem-initializer: tstring(tstring&& s) : std::string(std::move(s)) {} now prefers to use the new constructor: basic_string(const _Tp& __t, const _Alloc& __a = _Alloc()) because tstring is convertible to string_view. This turns a non-allocating move into an allocating copy.
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