Re: std::string add nullptr attribute

[Jonny Grant <jg@jguk.org>]

On 20/02/2023 10:26, Xi Ruoyao 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.

Thank you for your reply.

As you say, throwing logic_error seems rational if a NULL gets through to the constructor; if standard didn't imply creating an empty std::string when NULL was passed through.

> 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.

Thank you for the suggestion, I gave that nonnull attribute a try, but it doesn't appear to warn for this example.

https://godbolt.org/z/boqTj6oWE

It should give a warning, as -fanalyzer enables -Wanalyzer-null-argument
https://gcc.gnu.org/onlinedocs/gcc/Static-Analyzer-Options.html


My preference would be to not have that char b = *str; maybe I would just do it within a macro enabled by a specific build 
Just to share my first example, with that char b = *str; inside a macro.
https://godbolt.org/z/9Wo6zY3rT

Kind regards
Jonny