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For the following classes, why is the output 1 instead of 10?:

class Top
{
public:
Top():a(1){}
Top(int arg):a(arg){}
virtual ~Top(){}
int a;
};

class Left : virtual public Top
{
public:
Left():b(2){}
int b;
};

class Right : virtual public Top
{
public:
Right(): c(3), Top(10){}
int c;
};

class Bottom : public Left, public Right
{
public:
Bottom():d(4){}
int d;
};

int main()
{
Bottom* bottom = new Bottom();
Right *right = bottom;
cout << "right->a: " << right->a << endl;

return 0;
}

wrote:
> For the following classes, why is the output 1 instead of 10?:
....

>
> class Right : virtual public Top
> {
> public:
> Right(): c(3), Top(10){}
> int c;
> };
>
> class Bottom : public Left, public Right
> {
> public:
> Bottom():d(4){}
> int d;
> };
>
> int main()
> {
> Bottom* bottom = new Bottom();
> Right *right = bottom;
> cout << "right->a: " << right->a << endl;
>
> return 0;
> }
>

The virtual base class constructor must be called
from the most derived constructor. In this case it must
be called from Bottom.

John.

John Grabner wrote:
> wrote:
> > For the following classes, why is the output 1 instead of 10?:
> ...
>
> >
> > class Right : virtual public Top
> > {
> > public:
> > Right(): c(3), Top(10){}
> > int c;
> > };
> >
> > class Bottom : public Left, public Right
> > {
> > public:
> > Bottom():d(4){}
> > int d;
> > };
> >
> > int main()
> > {
> > Bottom* bottom = new Bottom();
> > Right *right = bottom;
> > cout << "right->a: " << right->a << endl;
> >
> > return 0;
> > }
> >
>
> The virtual base class constructor must be called
> from the most derived constructor. In this case it must
> be called from Bottom.
>
> John.

I know this is a bit of a digression from the OP's question but
shouldn't there be a delete Bottom; to avoid a memory leak? (I'm not
completely sure -- this is a genuine question, not a rhetorical one.)

Paul Epstein

wrote:
> I know this is a bit of a digression from the OP's question but
> shouldn't there be a delete Bottom; to avoid a memory leak? (I'm not
> completely sure -- this is a genuine question, not a rhetorical one.)
>
> Paul Epstein

In my opinion, yes and no. Yes, the memory needs to be deleted, but no,
not by delete. Here would be my main:

int main()
{
boost::shared_ptr<Right> right(boost::shared_ptr<Bottom>(new
Bottom()));
cout << "right->a: " << right->a << endl;

return 0;
}

The shared_ptr cleans up for you when it goes out of scope.

Todd Gardner

> The virtual base class constructor must be called
> from the most derived constructor. In this case it must
> be called from Bottom.
>
> John.

So what happens to the explicit call to Top(10) in the initializer list
of class Right? Does it get called at all?