TITLE: Why is virtual base class initialization the way it is?


PROBLEM: hanlon@birch307.cray.com (Jim Hanlon)

I ran headlong into the behavior dictated by the paragraph on page 293 of
ARM section 12.6.2 and am looking for clues into *why* virtual base class
initialization is specified as it is?
 ...

My question is "Why?"  This specification seems to blow all hope of
encapsulation out of the water, since E must know all the details of it's
derivation graph. I can't see the need or benefit in this form of
initialization.



[ The complaint is that virtual base classes must have their constructors
  called explicitly by all direct AND INDIRECT descendants. -adc ]



RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 10 Apr 95

The initialization rules pretty much fall out of other requirements.
1. A base-class subobject must be initialized before its derived-class
   subobjects.
2. An object contains only one copy of a virtual base class no matter
   how many times it appears in the inheritance graph.
3. An intermediate base class doesn't know about its descendants or siblings.
4. Two intermediate base class constructors might have different ideas
   about the initialization of what they don't know is a shared base
   class.

Example:
	class VB { ... VB(int); ... };
	class A : virtual public VB {
		A() : VB(10) { ... } // A knows nothing about B
	};
	class B : virtual public VB {
		B() : VB(20) { ... } // B knows nothing about A
	};
	class C : public A, public B {
		C() { ... } // Is VB initialized with 10 or with 20?
	};
The contructors of A, B, and C are all allowed to assume that VB is
initialized, so VB must be initialized first. A and B don't know about
each other, and conflict on initializatin of VB. The necessary information
comes together only at a C constructor.

Resolving all these issues either leads to a language different from C++,
or to the C++ rules we have, or to a mess, as far as I can see. (One could
argue that we have a mess anyway, of course :-) In particular, one of
the desired features of multiple inheritance is the ability to combine
different classes that have no knowledge of or dependence on one another.

There is a clean solution, as John Skaller points out. A virtual base class
should probably contain no data, and should have no constructors that require
parameters (except for the copy ctor). Such a class is used only to define an
interface. Initialization of such virtual base classes is not an issue.