Re: RTTI names for classes in anonymous namespaces

[Jonathan Wakely <jwakely.gcc@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 1827 bytes --]

On Wed, 23 Aug 2023, 09:41 Varun Kumar E via Libstdc++, <
libstdc++@gcc.gnu.org> wrote:

>  hello all,
>
>       The RTTI names generated by gcc for classes in anonymous namespaces
> begin with the prefix asterisk(*).
>       Once the __name pointer is different why do we not return
> immediately? Instead we perform a string comparison if the name does not
> begin with asterisk(*).
>

This would have been more more useful as a reply to your previous email on
the subject (which I already replied to) rather than a separate thread.

When using shared objects there is no guarantee that duplicate RTTI data
gets combined, so we can have two or more copies of the data for a single
type. Using strcmp to compare the names works correctly in this case.

For types in anonymous namespaces (or types with no linkage) the type
cannot exist in two different shared objects, so we know that the RTTI data
is unique. That means a pointer comparison is ok.


      Could you please explain the reasoning behind this.
>
>       code snippet below:
>
>    type_info::operator==(const type_info& __arg) const _GLIBCXX_NOEXCEPT
>   {
>       if (std::__is_constant_evaluated())
>          return this == &__arg;
>
>       if (__name == __arg.__name)
>          return true;
>
>       #if !__GXX_TYPEINFO_EQUALITY_INLINE
>          // ABI requires comparisons to be non-inline.
>          return __equal(__arg);
>      #elif !__GXX_MERGED_TYPEINFO_NAMES
>          // Need to do string comparison.
>          return __name[0] != '*' && __builtin_strcmp (__name, __arg.name
> ())
> == 0;
>       #else
>          return false;
>       #endif
>      }
>
>      Code link: typeinfo - gcc-mirror/gcc - Sourcegraph
> <
> https://sourcegraph.com/github.com/gcc-mirror/gcc/-/blob/libstdc++-v3/libsupc++/typeinfo?L205
> >
>
>
> regards,
> Varun
>