Virtual base class constructor

Hello everyone,


From MSDN virtual base class reference, what means "If a derived class overrides a virtual function that it inherits from a virtual base class, and if a constructor or a destructor for the derived base class calls that function using a pointer to the virtual base class, the compiler may introduce additional hidden "vtordisp" fields into the classes with virtual bases."

I do not understand why an additional hidden filed is needed?

http://msdn2.microsoft.com/en-us/lib...DownFilterText


thanks in advance,
George

You'll have to ask Microsoft that one. This question is specific to the implementation of Microsoft's C++ compilers, and is outside the scope of C++ as a language.

Sorry, grumpy!


I will post to Windows Programming sub-forum. :-)

Quote:

---

Originally Posted by grumpy

You'll have to ask Microsoft that one. This question is specific to the implementation of Microsoft's C++ compilers, and is outside the scope of C++ as a language.

---

regards,
George

It is not a Windows programming question since it applies generally, but is a Microsoft specific implementation detail.

Thanks laserlight,


I have made a sample below and the output is Derived1. I have monitored that the content of this object in the constructor of class Derived1 is,

My confusion is, why an additional hidden "vtordisp" fields exists? What is the issue described by the quoted statements in my original question?

- this 0x0022fc4c {j=-858993460 } Derived1 * const
- Base {i=100 } Base
- __vfptr 0x0109783c const Derived1::`vftable' *
[0] 0x01091037 [thunk]:Derived1::foo`vtordisp{4294967292,0}' (void) *
i 100 int
j -858993460 int

Code:

---

#include <iostream>

using namespace std;

class Base {
private:
int i;
public:
Base()
{
i = 100;
}

virtual void foo()
{
cout << "Base " << endl;
}
};

class Derived1 : virtual public Base {
private:
int j;
public:
Derived1()
{
Base* pb = this;
pb->foo(); // call Base class's virtual function inside derived class's
constructor by using pointed to base type
j = 200;
}

virtual void foo()
{
cout << "Derived1 " << endl;
}
};

class Derived2 : virtual public Base {
private:
int k;
public:
Derived2()
{
k = 300;
}

virtual void foo()
{
cout << "Derived2 " << endl;
}
};

class Final : public Derived1, public Derived2 {
private:
int t;
public:
Final()
{
t = 400;
}

virtual void foo()
{
cout << "Final " << endl;
}
};

int main()
{
Final f;

return 0;
}

---

Quote:

---

Originally Posted by laserlight

It is not a Windows programming question since it applies generally, but is a Microsoft specific implementation detail.

---

regards,
George