libstdc++ Chapter 28. Demangling example

[Jonny Grant]

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

Jonny