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

"Alf P. Steinbach" <>
Tue, 03 Jan 2012 00:46:31 +0100
On 02.01.2012 21:16, 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() {}

Use virtual inheritance for the interface (that is for `Observer`).

struct Shape : public virtual Observer

struct Square : public Shape, public ObserverImp

Technically OK.

Shape* ShapeFactory()

Please use different naming conventions for types and functions.

   return new Square;

No. Forget the Java-isms. You don't need any
factory-manager-singleton-envelope-blahblah complication.

Or at least, if you absolutely have to code Java in C++, then do it

E.g., then return a smart pointer, not a raw pointer.

$ 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++?

See above.

It then compiles nicely, and corresponds directly to Java-like
implementation inheritance for an interface.

If you run into any problems then please post to a new thread with your
exact code and compiler invocation.

Cheers & hth.,

- Alf

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