On 10/02/2023 22:03, Jonathan Wakely wrote: > On Fri, 10 Feb 2023 at 21:30, Jonny Grant <jg@xxxxxxxx> 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"; Sharing my example, gives compile error for 0, nullptr but not NULL (only for dumb direct calls) : // g++ -std=c++23 -Wall -O1 -o string3 string3.cpp #include <iterator> #include <string> void nullptr_compile_abort() __attribute__((error("nullptr compile error"))); class str_string { public: str_string(nullptr_t) { nullptr_compile_abort(); } str_string(int) { nullptr_compile_abort(); } str_string(void *) { nullptr_compile_abort(); } }; int main() { str_string y(nullptr); } >> >> >> g++ -std=c++23 -Wall -O1 -o string2 string2.cpp >> In constructor ‘str_string::str_string(nullptr_t)’, >> inlined from ‘int main()’ at string2.cpp:48:25: >> string2.cpp:20:50: error: call to ‘nullptr_compile_abort’ declared with attribute error: nullptr compile error >> 20 | str_string(nullptr_t) { nullptr_compile_abort(); } >> >> Jonny Maybe C++ guidelines not_null is a better approach to prevent construction? I've not tried it yet. Jonny
