Class sizes with virtual inheritance in C++

Question

#include

using namespace std;

class abc
{
    int a;
};
class xyz : public virtual abc
{
    int b;
};

int main()
{
    abc obj;
    xyz obj1;
            


        

Answer 1

Virtual bases classes increase (dynamic, runtime) conversion requirements, and I suppose that the size increase are for a kind of 'pivoting ground' to walk the (base) class hierarchy in non-ambiguous ways when doing such conversions.

In less mumbo jumbo, here is a counter example that could show what's going on:

• Using virtual abc http://ideone.com/h5y7R
  • sizeof(xyz) == 44 (88 on 64bit arch)
• Using nonvirtual abc http://ideone.com/h5y7R
  • sizeof(xyz) == 68 (128 (padded) on 64bit arch)

 

#include

class abc
{
    int x;
    virtual void t();
};

template  struct interm : virtual abc 
{
    virtual void t();
    virtual void s();
};

struct xyz : 
    /*virtual*/ interm<1>, 
    /*virtual*/ interm<2>, 
    /*virtual*/ interm<3>, 
    /*virtual*/ interm<4>,
    /*virtual*/ interm<5>, 
    /*virtual*/ interm<6>, 
    /*virtual*/ interm<7>, 
    /*virtual*/ interm<8>
{
    int b;
    virtual void t();
    virtual void s();
};


int main()
{
    std::cout << sizeof(abc)       << std::endl;
    std::cout << sizeof(interm<1>) << std::endl;
    std::cout << sizeof(xyz)       << std::endl;
    return 0;
}

You'll notice a significant reduction in size when marking the abc base as virtual (at least on gcc). Also, note no effect when marking (any) of the intermediate base classes as (non)virtual.