virtual_inheritance_difference.cpp

#include <algorithm>
#include <cctype>
#include <fstream>
#include <iostream>
#include <iterator>
#include <set>
#include <string>

using namespace std;

class Furniture {
public:
  Furniture() {
    _price = 0;
    std::cout << __FUNCTION__ << " ctor" << std::endl;
  }
  virtual ~Furniture() { cout << "Furiture dtor" << endl; }
  virtual int Price() { return _price; }

protected:
  int _price;
};

class Sofa : virtual public Furniture {
public:
  Sofa() : _color(2) {
    _price = 1;
    std::cout << __FUNCTION__ << " ctor" << std::endl;
  }
  virtual ~Sofa() { cout << "sofa dtor" << endl; }
  virtual int Color() { return _color; }
  virtual void SitDown() { cout << "Sofa sitdown" << endl; }

protected:
  int _color;
};

class Bed : virtual public Furniture {
public:
  Bed() : _length(4), _width(5) {
    _price = 3;
    std::cout << __FUNCTION__ << " ctor" << std::endl;
  }
  virtual ~Bed() { cout << "Bed dtor" << endl; }
  virtual int Area() { return _length * _width; }
  virtual void Sleep() { cout << "Bed sleep" << endl; }

protected:
  int _length;
  int _width;
};

class SofaBed : public Sofa, public Bed {
public:
  SofaBed() : _height(6) { std::cout << __FUNCTION__ << " ctor" << std::endl; }
  virtual ~SofaBed() { cout << "SofaBed dtor" << endl; }
  virtual void SitDown() { cout << "SofaBed sitdown" << endl; }
  virtual void Sleep() { cout << "SofaBed sleep" << endl; }
  virtual int Height() { return _height; }

protected:
  int _height;
};

int main() {
  SofaBed sbed;
  cout << sizeof(Furniture) << endl;
  cout << sizeof(Sofa) << endl;
  cout << sizeof(Bed) << endl;
  cout << sizeof(sbed) << endl;
  return 0;
}

/*
output from gcc 5.4 x86_64:

Furniture ctor
Sofa ctor
Bed ctor
SofaBed ctor
16
32
32
56
SofaBed dtor
Bed dtor
sofa dtor
Furiture dtor

*/

Object ABI:

$3 = {
  <Sofa> = {
    <Furniture> = {
      _vptr.Furniture = 0x401418 <vtable for SofaBed+160>, 
      _price = 3
    }, 
    members of Sofa: 
    _vptr.Sofa = 0x401390 <vtable for SofaBed+24>, 
    _color = 2
  }, 
  <Bed> = {
    members of Bed: 
    _vptr.Bed = 0x4013d8 <vtable for SofaBed+96>, 
    _length = 4, 
    _width = 5
  }, 
  members of SofaBed: 
  _height = 6
}


0x7fffffffddb0:	0x00401390	0x00000000	0x00000002	0x00000000
0x7fffffffddc0:	0x004013d8	0x00000000	0x00000004	0x00000005
0x7fffffffddd0:	0x00000006	0x00000000	0x00401418	0x00000000
0x7fffffffdde0:	0x00000003	0x00000000	0x9d2d9400	0xa96fa29b
0x7fffffffddf0:	0xffffdee0	0x00007fff	0x00000000	0x00000000

From the memory layout, we can see that the underlay object model is:
_vptr.Sofa
_color.Sofa
_vptr.Bed
_length.Bed
_width.Bed
_height.SofaBed
_vptr.Furniture
_price.Furniture