libstdc++ Chapter 28. Demangling example

[Jonathan Wakely]

On Tue, 18 Apr 2023 at 23:50, Jonny Grant <jg@jguk.org> wrote:
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>
>
> On 18/04/2023 15:49, Jonathan Wakely wrote:
> > On Tue, 18 Apr 2023 at 14:00, Jonny Grant <jg@jguk.org> wrote:
> >>
> >>
> >>
> >> On 12/04/2023 17:07, Jonny Grant wrote:
> >>>
> >>>
> >>> On 12/04/2023 16:36, Jonathan Wakely wrote:
> >>>> On Wed, 12 Apr 2023 at 16:10, Jonny Grant <jg@jguk.org> wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>> On 04/04/2023 12:26, Jonathan Wakely wrote:
> >>>>>> On Fri, 31 Mar 2023 at 21:38, Jonny Grant wrote:
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>> On 31/03/2023 17:18, Jonathan Wakely wrote:
> >>>>>>>> On Fri, 31 Mar 2023 at 16:24, Jonny Grant <jg@jguk.org> wrote:
> >>>>>>>>>
> >>>>>>>>> Hello
> >>>>>>>>>
> >>>>>>>>> I tried out this code. Maybe I'm doing something wrong?
> >>>>>>>>>
> >>>>>>>>> https://godbolt.org/z/Y78h4f1W6
> >>>>>>>>>
> >>>>>>>>> https://gcc.gnu.org/onlinedocs/gcc-12.2.0/libstdc++/manual/manual/ext_demangling.html
> >>>>>>>>>
> >>>>>>>>> However only get reduced output after status -2 is returned:
> >>>>>>>>>
> >>>>>>>>> std::bad_exception      =>
> >>>>>>>>>
> >>>>>>>>> Looks like std::cout gets broken by the nullptr realname (that's a frustration). So I put an if(realname) in the godbolt link above.
> >>>>>>>>
> >>>>>>>> N.B. passing a null pointer to printf("%s", ptr) is undefined.
> >>>>>>>> Printing "(null)" is not required by the standards.
> >>>>>>>>
> >>>>>>>> Anyway, the exception classes haven't printed their mangled name for
> >>>>>>>> many many years, if they ever did. Only the second part of the
> >>>>>>>> example, using typeid, is valid.
> >>>>>>>
> >>>>>>> Ok I see. Is it better that the std::bad_exception part of the example is removed?
> >>>>>>
> >>>>>> Yes, done for GCC 13.
> >>>>>
> >>>>> Great!
> >>>>>
> >>>>>>>
> >>>>>>> c++filt doesn't demangle St13bad_exception either.
> >>>>>>
> >>>>>> c++filt needs the _Z prefix that identifies a mangled C++ name. The
> >>>>>> __cxa_demangle function doesn't need that.
> >>>>>>
> >>>>>> $ c++filt St13bad_exception
> >>>>>> St13bad_exception
> >>>>>> $ c++filt _ZSt13bad_exception
> >>>>>> std::bad_exception
> >>>>>
> >>>>> Ok that makes sense.
> >>>>>
> >>>>>>>>> Maybe it's also a good time to update the example abi::__cxa_demangle call from those 0 parameters to be NULL?
> >>>>>>>>
> >>>>>>>> Maybe nullptr.
> >>>>>>>
> >>>>>>> ok
> >>>>>>
> >>>>>> I didn't change them, using 0 as a null pointer constant seems OK.
> >>>>>
> >>>>> Isn't nullptr, or at least NULL better? NULL has always been used in C++ code, I've seldom seen 0 used professionally.
> >>>>
> >>>> NULL is a macro, that requires inclusion of a header. nullptr is
> >>>> superior, but not available in C++98. 0 works OK here.
> >>>
> >>> Fair enough. One of those headers must have included <cstddef> - I recall it did compile with NULL.
> >>>
> >>
> >> https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#es47-use-nullptr-rather-than-0-or-null
> >> ES.47: Use nullptr rather than 0 or NULL
> >>
> >> Where there's a 0 in an example, it gets copy-pasted. If the example use nullptr, that's what is copy-pasted.
> >
> > Yes, exactly, and then it doesn't compile with -std=c++98
>
> Isn't it reasonable for GCC13 examples to be based on c++11 these days? c++98 is already 25 years old.
>
> Alas, the example doesn't compile with -std=c++98 due to missing
> #include <cstdlib>
>
> https://gcc.gnu.org/onlinedocs/libstdc++/manual/ext_demangling.html
>
> https://godbolt.org/z/KjKfboqxG
>
> This documentation for abi::__cxa_demangle  says both these can be NULL, it doesn't say 0. Feels better to follow the manual and change them to NULL.
>
> char *  output_buffer,
> size_t *        length,
>
> https://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.3/a01696.html

Fine, I'll change them. Can I focus on more important things now?