Re: std::string add nullptr attribute

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Fri, 10 Feb 2023 at 22:38, Jonny Grant <jg@jguk.org> wrote:
>
>
>
> On 10/02/2023 22:03, Jonathan Wakely wrote:
> > On Fri, 10 Feb 2023 at 21:30, Jonny Grant <jg@jguk.org> wrote:
> >>
> >>
> >>
> >> On 09/02/2023 17:52, Jonathan Wakely wrote:
> >>> On Thu, 9 Feb 2023 at 16:30, Xi Ruoyao wrote:
> >>>>
> >>>> On Thu, 2023-02-09 at 14:56 +0000, Jonathan Wakely via Gcc-help wrote:
> >>>>>> Note, my code isn't like this, it is just an example to suggest
> >>>>>> adding the nullptr attribute, as its clearly already rejected at
> >>>>>> runtime.
> >>>>>
> >>>>> I assume you mean the nonnull attribute. That was added in 2020 and
> >>>>> then reverted because it broke some things:
> >>>>
> >>>> I remember I'd once made the same mistake when I suggested to add
> >>>> nonnull for ostream::operator<<(const string &) and I was lectured:
> >>>> nonnull is not only a diagnostic attribute, it also allows the compiler
> >>>> to assume the parameter is never null and rendering std::string(nullptr)
> >>>> an undefined behavior.
> >>>
> >>> Yes, I think that's what might have happened with the std::string change.
> >>
> >> My apologies, Jonathan, Xi, yes it is the __attribute__((nonnull)); I was mistaken to type as nullptr.
> >>
> >> I re-read, and it does seem nonnull is really an optimization that as a side effect may give some warnings. So I'm going to stop using it.
> >> https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
> >>
> >> (there is a typo in that manual section saying "nonnul" - I don't know if you have a moment to make a change in git? I didn't get replies on gcc-patches to my patches...)
> >>
> >> I searched and see like someone investigated this problem and saw it removed NULL checks http://www.rkoucha.fr/tech_corner/nonnull_gcc_attribute.html
> >>
> >> I saw wget2 removed the nonnull attribute due to the optimizer removing checks against NULL too
> >> https://gitlab.com/gnuwget/wget2/-/issues/200
> >>
> >>>> Then the example may just silently continue to run, instead of throwing
> >>>> an exception.  It would be an ironic example: an attempt to improve
> >>>> diagnostic finally made diagnostic more difficult.
> >>>
> >>> Indeed.
> >>>
> >>> Maybe we can add __attribute__((access(read, 1))) instead, which says
> >>> that we will read from the pointer, which also implies it must be
> >>> non-null.
> >>
> >> I tried this with gcc 12, as read_only, but it didn't stop when compiling. Maybe you have an example that demonstrates please?
> >>
> >> void f(const char * p) __attribute__((access(read_only, 1)));
> >>
> >>>
> >>> N.B. in C++23 string(nullptr) produces an error, although
> >>> string((const char*)nullptr) doesn't, so in practice it only prevents
> >>> the dumbest calls with a literal 'nullptr' token, and not the more
> >>> realistic problems where you have a pointer that happens to be null.
> >>
> >> That's good it stops compiling, the error is not that clear "use of deleted function" for me though.
> >>
> >> string.cpp: In function ‘int main()’:
> >> string.cpp:13:26: error: use of deleted function ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::nullptr_t) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::nullptr_t = std::nullptr_t]’
> >>    13 |     std::string c(nullptr);
> >>
> >>
> >>
> >>
> >> I made my own test class str_string which stops the build a different way. It only works if the dumbest calls with 'nullptr' as you found in your test.
> >>
> >> void nullptr_compile_abort() __attribute__((error("nullptr compile error")));
> >>
> >> str_string(nullptr_t) { nullptr_compile_abort(); }
> >
> > This doesn't work because std::is_constructible_v<std::string,
> > std::nullptr_t> would be true, and we want it to be false.
>
> Hmm, for me, this output is 0.
>   std::cout << std::is_constructible_v<std::string,std::nullptr_t> << "\n";

For C++23, yes, but if you add a constructor like your
str_string(nullptr_t) it would become 1.

Using a deleted function is observably different to using a
constructor that then produces an error when called.