TITLE: dominance and virtual inheritance

(Newsgroups: comp.lang.c++.moderated, 22 Sep 96)

CLAMAGE: clamage@taumet.Eng.Sun.COM (Steve Clamage)

[...]
Thus, many C++ experts recommend that a class used as a virtual base
class have no user-defined constructors at all, and no data either.
[...]
 

HOWLETT: howlett@netcom.com (Scott Howlett)

And this usually obviates the need for the virtual inheritance, no?
 

MARTIN: rmartin@oma.com (Robert C. Martin)

Not always.  The dominance rule still makes virtual inheritance of abstract
base classes useful.

class B
{
  public:
    virtual void f() = 0;
    virtual void g() = 0;
};

class D1 : public virtual B
{
  public:
    virtual void f(); // implemented.
};

class D2 : public virtual B
{
  public:
    virtual void g(); // implemented.
};

Now, the dominance rule allows this:

class M : public D1, public D2
{
}; 

M is a concrete class in which M::f is implemented by D1 and M::g is 
implemented gy D2.  Had B not been a virtual base, then M would have
had to be written to delegate to the appropriate functions:

class M, public D1, public D2
{
  public:
    virtual void f() {D1::f{};}
    virtual void g() {D2::g();}
};