Re: std::string add nullptr attribute

[Jonny Grant <jg@jguk.org>]

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";


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