Re: std::string add nullptr attribute

[Jonny Grant <jg@jguk.org>]

On 20/02/2023 10:37, Jonathan Wakely wrote:
> On Mon, 20 Feb 2023 at 10:26, Xi Ruoyao <xry111@xry111.site> wrote:
>>
>> On Sun, 2023-02-19 at 21:33 +0000, Jonny Grant wrote:
>>
>>> I noticed -Wanalyzer-null-dereference reports at build time a
>>> dereference. Also works if a function parameter. I wondered why
>>> std::string isn't detected by this static analyser option.
>>
>> Because the analyzer does not know the C++ standard disallows to use
>> NULL here.  It just analyzes the code.  The code in libstdc++ reads:
>>
>>       basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
>>       : _M_dataplus(_M_local_data(), __a)
>>       {
>>         // NB: Not required, but considered best practice.
>>         if (__s == 0)
>>           std::__throw_logic_error(__N("basic_string: "
>>                                        "construction from null is not valid"));
>>         const _CharT* __end = __s + traits_type::length(__s);
>>         _M_construct(__s, __end, forward_iterator_tag());
>>       }
>>
>> As you can see yourself, though the standard implies using NULL here is
>> a UB, libstdc++ does not really code a UB here.  So the analyzer will
>> consider the code absolutely valid.
> 
> Right, it's defined behaviour in libstdc++, as an extension.
> 
>>
>> Note that throwing a C++ exception is not a programming error.  It's
>> perfectly legal to catch the exception elsewhere.  It's also perfectly
>> legal not to catch it and treat it as an abort() (calling abort is also
>> not a programming error).
>>
>>
>>> It's not pretty, but this wrapper catches NULL passed at compile time:
>>>
>>> std::string make_std_string(const char * const str)
>>> {
>>>     // This line ensures: warning: dereference of NULL '0' [CWE-476]
>>> [-Wanalyzer-null-dereference]
>>>     char b = *str;
>>
>> You are invoking an undefined behavior here if str is NULL, so it's
>> essentially same as using a nonnull attribute for make_std_string.
> 
> And turned defined behaviour back into UB. The warning isn't reliable
> (only if the compiler can see the point is null, which isn't the case
> without optimization, or if the pointer comes from some non-inline
> function), the exception is. You're trading guaranteed exception for a
> not guaranteed warning and unbounded misoptimization due to undefined
> behaviour.
> 
> Even if this was a robust solution, is it really a problem that needs
> to be solved?

Feels useful to get build warnings if compiler knows nullptr is going to be dereferenced, as clang does.

Personally I feel runtime should equally handle possible nullptr by constructing strings in a try catch block so any exceptions are handled or logged at least...

Personally I would be pleased if GCC had a warning I could enable to report any logic_error exceptions it knew would execute.

Regards, Jonny