virtual table

virtual table

[Riccardo Manfrin]

Hi,

suppose I have three classes Foo, Bar, Zoo.

Foo inherits from Bar,
Bar inherits from Zoo.

Bar and Zoo are abstract classes (as defined here : 
http://en.wikibooks.org/wiki/C%2B%2B_Programming/Classes/Abstract_Classes)

The size of Foo shall account for the two pointers to Bar an Zoo vtables.

I was wondering if there is a way to have the Zoo vtable reference 
pointer embedded in the Bar vtable, so that Foo only has to store one 
pointer, while the second vtable reference is resolved when accessing 
the first. I'm aware that there are motivations like casting or multiple 
inheritance behind having all vtables pointers attached to the object 
instance in "ram" memory, rather then within the vtable "in rom", but I 
was wondering if there was a more "ram-efficient" way to handle this (as 
the vtables and the way the work is not dictated by the standard), which 
gcc could support.

R

Re: virtual table

[Ian Lance Taylor]

On Wed, Nov 20, 2013 at 6:53 AM, Riccardo Manfrin
<riccardomanfrin@gmail.com> wrote:
>
> suppose I have three classes Foo, Bar, Zoo.
>
> Foo inherits from Bar,
> Bar inherits from Zoo.
>
> Bar and Zoo are abstract classes (as defined here :
> http://en.wikibooks.org/wiki/C%2B%2B_Programming/Classes/Abstract_Classes)
>
> The size of Foo shall account for the two pointers to Bar an Zoo vtables.

For this kind of discussion it helps to show actual code.  If I
understand your description correctly, then your statement about the
size of Foo is incorrect.

#include <iostream>

class Zoo {
  virtual void zoo() = 0;
};

class Bar : public Zoo {
  virtual void bar() = 0;
};

class Foo : public Bar {
  void* v;
};

int
main()
{
  std::cout << sizeof(Foo) << std::endl;
}


On my x86_64 system this prints 16: 8 for the single vtable pointer, 8
for the void* field.  In other words, although Foo inherits from two
classes as you suggest, there is still only one vtable pointer.

Ian