Question

The following code are from the book “Inside the C++ object model”

#include <iostream>  
using namespace std;
class X{};
class Y: public virtual X{};
class Z: public virtual X{};
class A: public Y, public Z{};

int main()
{
     cout<<sizeof(X)<<" "<<sizeof(Y)<<" "<<sizeof(Z)<<" "<<sizeof(A)<<endl;
     return 0;
}

In my computer(Windows, VS2010), the output is:

1 4 4 8

Here’re my questions

1, sizeof(X)=1

The book says when X type generate two instance, say xa and xb. the compile insert a byte into A so that xa and xb can have different address. I’m not quite understand the reasons.

2, sizeof(Y)=4

By using virtual inheritance, will we have an additional virtual pointer? I guess this might be different from virtual pointer in polymorphism. Can anyone give me the memory layout for Y?

Thank you!

Answer 1

1. compiler new one char when the class is empty, so it can generate different object
2. sizeof(Y)=4 because it’s virtual inheritance, construct will generate vptr table which is 4 bytes on 32-bit system
3. if you are using visual studio use /d1reportAllClassLayout in properties->C/C++/Command to generate object layout
   class Y object layout will be on Visual Studio:
4. book ‘Inside C++ object model’ by Stanley B. Lippman explained this extremely well

 
        class Y size(4):
            +---
            0     | {vbptr}
            +---
            +--- (virtual base X)
            +---

      Y::$vbtable@:
       0    | 0
       1    | 4 (Yd(Y+0)X)    

vbi: class offset o.vbptr o.vbte fVtorDisp
X 4 0 4 0