Re: std::string add nullptr attribute

[Xi Ruoyao <xry111@xry111.site>]

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.

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.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University