Hello all,
In a large project I'm participating in, a design dilemma have arrived. Adding
virtual inheritance would solve it beautifully on the source code level, but
a potential drawback is the speed penalty it brings. Measurement is always
the way of approaching performance questions, but since that sometimes can be
tricky, I thought of starting with getting a theoretical understanding of
virtual inheritance.
Consider this code:
//----------------------------------------------
class Shared
{
public:
Shared() : m_ref(0) { }
virtual ~Shared() { }
int refCount() const { return m_ref; }
void ref() { m_ref++; }
void deref()
{
if(m_ref) m_ref--;
if(!m_ref) delete this;
}
protected:
unsigned int m_ref;
};
class B : public virtual Shared {};
class C : public virtual Shared {};
class D : public Shared {};
class BC : public B, public C {};
class DerivedB : public B {};
//----------------------------------------------
Follows do plenty of questions:
* Is class DerivedB in anyway affected by that its sub class B is a public
virtual of Shared? And if so, how?
* What are the differences in memory usage between an instance of class D and
an instance of class B? (if any)
* What are the performance differences of invoking BCinstance->ref() compared
to Dinstance->ref()?
* What are the performance differences in constructing and destructing an
instance of class BC and D?
Speaking in general terms, what are the penalties of virtual inheritance?
Consider that a class previously was as class D, and then became class BC. How
would that affect performance? Hard to tell? Definitely a performance
disaster? Or perhaps that "the additional level of indirection has a slight
performance overhead, but it's a small price to pay"?[1]
Detailed replies, pointers to documentation, is much appreciated. A googling
showed relatively contradictory results on the performance of virtual
inheritance, so solid information would be a relief(explanations to why it is
as it is would also be interesting to hear).
Thanks in advance,
Frans
1.
http://www.devx.com/tips/Tip/12789
