Re: Why doesn't this multiple virtual inheritance code compile?
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
"ObserverImp"
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
properly.
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
'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++?
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