g++ -DFIX_DIAMOND_INHERITENCE=0 -fdump-class-hierarchy test.cpp

Class foo
   size=4 align=4
   base size=4 base align=4
foo (0x140d45620) 0

Vtable for bar
bar::_ZTV3bar: 5u entries
0     12u
8     (int (*)(...))0
16    (int (*)(...))(& _ZTI3bar)
24    bar::~bar
32    bar::~bar

VTT for bar
bar::_ZTT3bar: 1u entries
0     ((& bar::_ZTV3bar) + 24u)

Class bar
   size=16 align=8
   base size=12 base align=8
bar (0x140d45690) 0
    vptridx=0u vptr=((& bar::_ZTV3bar) + 24u)
  foo (0x140d45700) 12 virtual
      vbaseoffset=-0x00000000000000018

Vtable for test
test::_ZTV4test: 5u entries
0     16u
8     (int (*)(...))0
16    (int (*)(...))(& _ZTI4test)
24    test::~test
32    test::~test

Construction vtable for bar (0x140d45d90 instance) in test
test::_ZTC4test0_3bar: 5u entries
0     16u
8     (int (*)(...))0
16    (int (*)(...))(& _ZTI3bar)
24    bar::~bar
32    bar::~bar

VTT for test
test::_ZTT4test: 2u entries
0     ((& test::_ZTV4test) + 24u)
8     ((& test::_ZTC4test0_3bar) + 24u)

Class test
   size=24 align=8
   base size=16 base align=8
test (0x140f1f800) 0
    vptridx=0u vptr=((& test::_ZTV4test) + 24u)
  foo (0x140d45d20) 16 virtual
      vbaseoffset=-0x00000000000000018
  bar (0x140d45d90) 0
      primary-for test (0x140f1f800)
      subvttidx=8u
    foo (0x140d45d20) alternative-path

g++ -DFIX_DIAMOND_INHERITENCE=1 -fdump-class-hierarchy test.cpp

Class foo
   size=4 align=4
   base size=4 base align=4
foo (0x140d45620) 0

Vtable for bar
bar::_ZTV3bar: 5u entries
0     12u
8     (int (*)(...))0
16    (int (*)(...))(& _ZTI3bar)
24    bar::~bar
32    bar::~bar

VTT for bar
bar::_ZTT3bar: 1u entries
0     ((& bar::_ZTV3bar) + 24u)

Class bar
   size=16 align=8
   base size=12 base align=8
bar (0x140d45690) 0
    vptridx=0u vptr=((& bar::_ZTV3bar) + 24u)
  foo (0x140d45700) 12 virtual
      vbaseoffset=-0x00000000000000018

Vtable for test
test::_ZTV4test: 5u entries
0     16u
8     (int (*)(...))0
16    (int (*)(...))(& _ZTI4test)
24    test::~test
32    test::~test

Construction vtable for bar (0x140d45d90 instance) in test
test::_ZTC4test0_3bar: 5u entries
0     16u
8     (int (*)(...))0
16    (int (*)(...))(& _ZTI3bar)
24    bar::~bar
32    bar::~bar

VTT for test
test::_ZTT4test: 2u entries
0     ((& test::_ZTV4test) + 24u)
8     ((& test::_ZTC4test0_3bar) + 24u)

Class test
   size=24 align=8
   base size=16 base align=8
test (0x140f1f800) 0
    vptridx=0u vptr=((& test::_ZTV4test) + 24u)
  foo (0x140d45d20) 16 virtual
      vbaseoffset=-0x00000000000000018
  bar (0x140d45d90) 0
      primary-for test (0x140f1f800)
      subvttidx=8u
    foo (0x140d45d20) alternative-path

test.cpp

#if FIX_DIAMOND_INHERITENCE
#define VIRTUAL_INHER virtual
#else
#define VIRTUAL_INHER
#endif

struct foo 
{
int i;
};

struct bar : public VIRTUAL_INHER foo
{
int j;
virtual ~bar() {}
};

struct test : public VIRTUAL_INHER foo, public bar
{
int k;
};

int main()
{
 test t;
}