Re: Why doesn't this multiple virtual inheritance code compile?

From:

Robert Wessel <robertwessel2@yahoo.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 02 Jan 2012 16:23:35 -0600

Message-ID:

<rbb4g798tnielionlla5if5iucdvj9f17e@4ax.com>

On Mon, 2 Jan 2012 12:16:31 -0800 (PST), Chris Stankevitz
<chrisstankevitz@gmail.com> wrote:

My intention is to
- Create an abstract base class "Shape" that must be an "Observer"
- Create an class "Square" that is a "Shape" and also an
"ObserverImp"

I thought I could do this like so:

struct Observer
{
virtual void Notify() = 0;
};

struct ObserverImp : public Observer
{
void Notify() {}
};

struct Shape : public virtual Observer
{
};

struct Square : public Shape, public ObserverImp
{
};

Shape* ShapeFactory()
{
return new Square;
}

$ g++ -Wall -c test.cpp
test.cpp: In function 'Shape* ShapeFactory()':
test.cpp:21:14: error: cannot allocate an object of abstract type
'Square'
test.cpp:16:1: note: because the following virtual functions are
pure within 'Square':
test.cpp:3:16: note: virtual void Observer::Notify()
test.cpp:22:1: warning: control reaches end of non-void function

Q: How can I do this using c++?

Instantiating Square requires instantiating both an Observer and
ObserverImp under the hood. The latter is not a problem, but Observer
has a pure virtual function, thus cannot be instantiated without
providing an implementation. The obvious solution is to add an
implementation of the required function to Square:

void Observer::Notify() {};