maria-developers team mailing list archive

[Sergei Golubchik <serg@xxxxxxxxxxx>]

Hi, Nirbhay!

On May 18, Nirbhay Choubey wrote:
> >
> > Besides, this simply reading THD pointer from somewhere, while, in
> > fact, it's unambigously defined by 'this'.
> >
> > Can you use TABLE::in_use pointer?
> 
> I am not sure about it. There are some methods that use THD
> without directly accessing TABLE.
> 
> > Or that pointer arithmetics, even (as above).
> 
> I turned that into a macro, but its gives the following warning :
> 
> warning: invalid access to non-static data member
> ‘Open_tables_state::temporary_tables’  of NULL object [-Winvalid-offsetof]
>  #define CURRENT_THD ((THD *)(((char*)this) - offsetof(THD,
> temporary_tables)))
> 
> I believe offsetof is not considered safe for non-PODs?

Yes. It works de facto unless you use virtual inheritance,
and we don't use it yet. There is even a my_offsetof() macro that is
used to shut this warning up. Still, it's, of course, a hack, best
avoided.

> OTOH, I can change the class methods to accept THD * as a parameter.
> and that way we can get rid of caching this this redundant pointer. A
> bit ugly but works.

As far as I can see, you can either inherit THD from Temporary_tables
(and then use THD *thd=static_cast<THD*>(this)) or simply make your
Temporary_tables a part of THD, not a separate class.

In the second case you can still keep all the implementation in
temporary_tables.cc, but temporary_tables.h will be merged into sql_class.h

Regards,
Sergei
Chief Architect MariaDB
and security@xxxxxxxxxxx