Re: Virtual base class constructor

From:

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Wed, 26 Mar 2008 10:13:18 -0500

Message-ID:

<uSwT9N1jIHA.1132@TK2MSFTNGP06.phx.gbl>

George wrote:

Sorry, Ben!

What I quote is not what you mentioned, but what MSDN mentioned. Let
me ask in another way,

Here is the related statement from MSDN and here is my code.

1. Does my code reflects what MSDN mentioned?

Almost. I think the call to the virtual function should be made in a class
other than the first base class (so change to Final : Derived2, Derived1).

2. What is the purpose of "vtordisp" fields and how it is used?

Because Derived1 constructor and Derived2 constructors both have to be able
to get the address of the Base subobject, but it is shared between them.

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/library/wcz57btd.aspx#Mtps_DropDownFilterText

[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;
}
[/Code]

regards,
George