virtual inheritance question

[chrolson]

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;
}

 

[John Grabner]

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.

 

[pauldepstein]

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

 

[Todd Gardner]

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

 

[chrolson]

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?