Re: [PATCH PR gdb/16841] virtual inheritance via typedef cannot find base

[Simon Marchi <simon.marchi@ericsson.com>]

On 2018-07-20 07:16 PM, Weimin Pan wrote:
> Finding data member in virtual base class
> 
> This patch fixes the original problem - printing member in a virtual base,
> using various expressions, do not yield the same value. Simple test case
> below demonstrates the problem:
> 
> % cat t.cc
> struct base { int i; };
> typedef base tbase;
> struct derived: virtual tbase { void func() { } };
> int main() { derived().func(); }
> % g++ -g t.cc
> % gdb a.out
> (gdb) break derived::func
> (gdb) run
> (gdb) p i
> $1 = 0
> (gdb) p base::i
> $2 = 0
> (gdb) p derived::base::i
> $3 = 0
> (gdb) p derived::i
> $4 = 4196392
> 
> To fix the problem, the virtual-base offset, relative to its derived class,
> needs to be fetched, via baseclass_offset(), and used in calculating the
> address of its data member in value_struct_elt_for_reference().

Hi Weimin,

I have looked at this a little bit, but unfortunately I don't really understand
what's going on (maybe somebody else does and could review it?).  I understand
the issue, and can see that your patch fixes it, but I don't understand how it
does it.  It would help if you could walk us through your code.  It maybe also
means that some comments would be appropriate, to explain what's going on.

Thanks,

Simon