Re: Virtual inheritance
On May 1, 2:31 pm, "Massimo" <bar...@mclink.it> wrote:
"James Kanze" <james.ka...@gmail.com> ha scritto nel messaggionews:117801=
0711.348780.259290@n76g2000hsh.googlegroups.com...
I don't get what you're trying to say. If you write something
like:
class A : public B {} ;
B is NOT a virtual base. Period, and regardless of anything
else in the program. There will be one instance of B for every
instance of A, always.
That's exaclty what I would expect.
But then, if B has a virtual base C, and I do
class A1 : public B {};
class A2 : public B {};
class D: public A1,public A2 {};
As you said, B, is not a virtual base, and I should have two instances of=
B:
one in A1 and the other in A2.
You do.
Each of these *should* contain its own base
classes, up to and including C.
No. When you defined B, you said that you want all instances of
B in a single hierarchy to share the same C. That's what the
virtual means in inheritance. More generally, it means that
*all* classes which inherit virtually from C share the same
instance, regardless of where they might be in the hierarchy.
Why is it, then, that when I re-derive from
both of them the instance of C becomes unique?
Because you told the compiler that that's what you wanted.
One way of looking at it is that the virtual base class is in
fact provided by the most derived class. It is the most
derived class which will call its constructor, for example, and
it is the most derived class which will determine where it is
physically located. Even if the author of the most derived
class doesn't even know it is there. So when you inherit
virtually, you are more or less saying I want a single instance
to be provided by the most derived class.
This is what is confusing me.
Just curious, but what would you expect if in addition, D
derived virtually from C, i.e.:
class B : public virtual C {} ;
class A1 : public B {} ;
class A2 : public B {} ;
class D : public A1, public A2, public virtual C {} ;
Note that you can have addional instances of C:
class B : public virtual C {} ;
class A1 : public B, public C {} ;
class A2 : public B {} ;
class D : public A1, public A2, public virtual C {} ;
This hierarchy contains two instances of C, one for all of the
virtual inheritance, and one for A1.
If you want a somewhat clearer view, try starting with:
class C
{
public:
C( std::string const& i )
{
std::cout << "C initialized by " << i << std::endl ;
}
} ;
and experimenting with different hierarchies, each time adding
an initializer for C with the class name, e.g.:
B::B() : C( "B" ) {}
A1::A1() : C( "A1" ) {}
// ...
You'll find that:
-- there's no problem specifying the initializer, as long as
there is a virtual inheritance somewhere below the class in
the hierarchy -- you don't have to specify it in the class
doing the initialization --, and
-- the constructor for C will always (without exception) be
called from the most derived class.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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