Re: std::string add nullptr attribute

[Xi Ruoyao <xry111@xry111.site>]

On Mon, 2023-02-20 at 10:37 +0000, 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.

Well, maybe we should have a warning here with -Wpedantic (or something)
as the standard does not allow people to pass NULL and expect a
logic_error.  But "deliberately making a UB to raise the warning" is not
good.

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