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

Robert Wessel <>
Mon, 02 Jan 2012 16:23:35 -0600
On Mon, 2 Jan 2012 12:16:31 -0800 (PST), Chris Stankevitz
<> 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

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
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() {};

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