Test the vptr position in object model

result-g++.txt

sizeof(Base)=16
sizeof(Derived)=16
0x7fffd1303400
0x7fffd1303408
vptr is at the beginning? true

result-g.txt

sizeof(Base)=16
sizeof(Derived)=16
0x7fffd1303400
0x7fffd1303408
vptr is at the beginning? true

result-vs.txt

sizeof(Base)=8
sizeof(Derived)=8
0013FF70
0013FF74
vptr is at the beginning? true

vptr-position.cpp

#include <iostream>
#include <cstdio>
#include <cassert>

using namespace std;

void printP( const void* p)
{
    printf("%p\n", p);
}

class Base
{
public:
    virtual ~Base(){}
    int a;
};

class Derived: public Base
{
public:
    virtual ~Derived() {}
};

int main()
{
    cout << "sizeof(Base)=" << sizeof(Base) << endl;
    cout << "sizeof(Derived)=" << sizeof(Derived) << endl;

    Base b;
    const char* str = (reinterpret_cast<int*>(&b) == &b.a) ? "false" : "true";
    printP( reinterpret_cast<void*>(&b));
    printP(reinterpret_cast<void*>(&b.a));
    cout << "vptr is at the beginning? " << str << endl;
}

• Even both base and derived class have virtual functions, in derived class there is only 1 copy of vptr (in this case the base classes doesn't hold one)
• seems, g++ and vc++ implementation put the vptr at the begining, not the end