Re: Could GDB get offset of a field in virtual base class through NULL pointer

[hex <heixia108 at gmail dot com>]

2013/9/30 Jan Kratochvil <jan.kratochvil@redhat.com>:
> On Sun, 29 Sep 2013 03:59:54 +0200, hex wrote:
>> > I do not see what it should do.  In the following case &(((B *)&OBJECT)->a)
>> > prints once 12 and once 16 for different OBJECT so what it should print for 0?
>> >
>> > class X:public virtual A,public B {};
>> > class C {
>> > public:
>> >         int c;
>> > };
>> > class Y:public virtual A,public C,public B {};
>> > #include <iostream>
>> > int main() {
>> >         X x;
>> >         Y y;
>> >         std::cout << (char *)&(((B *)&x)->a)-(char *)&x << std::endl;
>> >         std::cout << (char *)&(((B *)&y)->a)-(char *)&y << std::endl;
>> > }
>> >
>>
>> If we use &(((B *)0)->a), we are likely to get offset of 'a' in class
>> B. If GDB could
>> support this specific case, we do not need a real object to get the offset.
>
> This would apply if you had s/virtual A/A/.  But with the inheritance of
> A being virtual the memory location of A inside the whole object instance is
> "random", it does not depend on B but it depends on X or Y.  Specifically it
> depends on virtual tables used for the specific instance, the virtual tables
> specify the location of A.  This is what I am trying to show you in the
> example above.
>
> The same expression (char *)&(((B *)&OBJECT)->a) produces different result
> depending on which OBJECT you pass there.  Therefore which result should
> produce passing 0 instead of &OBJECT there?  It cannot be a single number.
>
>
> Jan Kratochvil


Thank you for the explanation.

I hope &(((B *)0)->a) to be regarded as a special case that gets the
same value as (B object; (char *)&((&object)->a) -  (char *)&object).

If G++ emits A's offset in B to the program's DAWRF file, GDB could
support this case by saving the offset. But I checked, only found this
offset in the binary code of B's constructor function.