Re: ambiguity in diamond inheritance

From:

happyasaclam111@gmail.com

Newsgroups:

comp.lang.c++

Date:

Thu, 6 Mar 2008 19:37:51 -0800 (PST)

Message-ID:

<31e04143-5336-4dcb-b01f-3767ee252c02@s12g2000prg.googlegroups.com>

On Mar 6, 3:38 am, karthikbalaguru <karthikbalagur...@gmail.com>
wrote:

Hi,

I wonder why is there an ambiguity in diamond
inheritance when we try to call the method in the
parent class from the class derived from multiple
(two)childs of the parent class.
Why C++ does not handle it internally and give us the
path to access the method in the parent class ?
Any constraints ?

Thx in advans,
Karthik Balaguru

Let's say we have four classes: A, B, C, and D
B & C both derive from A
D derives from B & C

D essentially "contains" two A sub-objects. One for the B part and
one for the C part. Consider the following code:

#include <iostream>

class A
{
    int n;
public:
    A(int i) : n(i) {};
    virtual void f() { std::cout << "From A: " << n << std::endl; }
};

class B : public A
{
public:
    B() : A(30) {};
};

class C : public A
{
public:
    C() : A(50) {};
};

class D : public B, public C {};

int main()
{
    D d;
    d.C::f(); //prints 50
    d.B::f(); //prints 30
    return 0;
}

If both B & C had derived virtually from A:
class B : public virtual A {/*...*/};
class C : public virtual A {/*...*/};
Then D would only have one A sub-object. In this case, D would be
responsible for constructing A.