c++ - Why does this code crash at the places mentioned?

Question

Can you please elaborate why this code crashes at the places mentioned? I am a bit stumped on this. I guess that it has got something to do with sizeof(int) but I am not so sure. Can anybody explain?

class Base
{
public:
    virtual void SomeFunction() 
    {
        printf("test base
");
    }

    int m_j;
};

class Derived : public Base {
public:
   void SomeFunction() 
   {
       printf("test derive
");
   }

private:
   int m_i;
};

void MyWonderfulCode(Base baseArray[])
{
   baseArray[0].SomeFunction();  //Works fine
   baseArray[1].SomeFunction();  //Crashes because of invalid vfptr
   baseArray[2].SomeFunction();  //Crashes because of invalid vfptr
   baseArray[3].SomeFunction();  //Works fine
   baseArray[4].SomeFunction();  //Crashes because of invalid vfptr
   baseArray[5].SomeFunction();  //Crashes because of invalid vfptr
   baseArray[6].SomeFunction();  //Works fine
   baseArray[7].SomeFunction();  //Crashes because of invalid vfptr
   baseArray[8].SomeFunction();  //Crashes because of invalid vfptr
   baseArray[9].SomeFunction();  //Works fine
}
int _tmain(int argc, TCHAR* argv[])
{
   Derived derivedArray[10];
   MyWonderfulCode(derivedArray);
   return 0;
}

Answer 1

sizeof(Derived) is greater than sizeof(Base). That's the reason.

Generally, indexing an array of Foo objects at index i works the following way:

element_address = array_base_address + i * sizeof(Foo)

You can see how this indexing breaks if the array elements are not of expected size. Why it works for some indices is because sometimes the calculated element address points to a valid object in memory (but it's not actually the ith object).