virtual inheritance

Question

If I have something like

class Base1 {};

class Base2 {};

class Derived : public Base1, public Base2 {};

Then order of constructor call on making object of Derived is

Base1
Base2

i.e in the order they appear in

class Derived : public Base1, public Base2 {};

But If I change it to class Derived : public Base1, virtual public Base2 {}; Then Order of constructor call becomes

Base2
Base1

I am not able to understand why this is so ?

Another simple doubt: what can be the meaning and purpose of inheriting Base1 virtually.

11 months you're on SO and you still can't format properly your posts?! — BlackBear
– BlackBear, Commented Apr 24, 2012 at 12:53
I'm assuming you've got a typo, because you declare Base1 and Base2, but then use Base and Base1. — Cornstalks
– Cornstalks, Commented Apr 24, 2012 at 12:55

Lara Bailey · Accepted Answer · 2012-04-24 13:21:52Z

4

Those are just the rules. The most derived class being constructed initializes all the virtual base classes in the hierarchy first before starting the initialization of its non-virtual direct bases.

The rules come from the standard (ISO/IEC 14882:2011), section 12.6.2 [class.base.init] / 10.

A rationale for this approach would be that it ensures that all base classes are initialized before their derived classes whether or not they are virtual bases.

edited Apr 24, 2012 at 13:21

answered Apr 24, 2012 at 12:54

Lara Bailey

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David Rodríguez - dribeas · Accepted Answer · 2012-04-24 13:07:40Z

On the order of initialization, Charles has already answered properly: those are the rules, first the virtual bases in the order of declaration, then the non-virtual bases in the order of declaration, then member variables in the order of declaration.

What is the meaning of virtual inheritance? It means that your object derives from that particular base, but that if in a hierarchy more than one subobjects (bases of the complete type) inherit virtually from the same base type, only one base subobject will be present. You can read the virtual keyword there as I am willing to share my base with other objects in the complete type.

struct ubase {};
struct sbase {};
struct A : ubase, virtual sbase {};   // has a sbase subobject, but is willing to share..
struct B : ubase, virtual sbase {};   // ... but want to have my own ubase
struct C : A, B {};                   // only one sbase subobject, shared by A and B
                                      // ... but two ubase subobjects, A::ubase and B::ubase

As per the second question, when do you want to use virtual inheritance? Whenever in a type hierarchy you might end up inheriting from the same base more than once and in your design all those occurrences of the base class are just one. In general, it is quite uncommon to use virtual inheritance other than in a few particular cases.

ManiP · Accepted Answer · 2012-04-24 12:59:50Z

1

As for inheriting virtually. It comes into play when you have a derived class like below

class A {};
class B : public virtual A {} ;
class C : public virtual A {} ;
class D : B, C { }  // this class has only one instance of A!

aka the diamond problem!

answered Apr 24, 2012 at 12:59

ManiP

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