Re: Non-default ctors, virtual base classes, & multiple inheritance all at once!

[sidster <patrick at mail dot boxsoft dot com>]

Greetings,


* Alexandre Oliva (aoliva@redhat.com) [20000906 07:45]:
> On Sep  5, 2000, Benjamin Scherrey <scherrey@switchco.com> wrote:
> 
> > I can pass a parameter up the tree to the virtual base class, in
> > others, the default ctor is always called even though my code
> > clearly specifies the ctor taking a parm.  Is this a compiler bug or
> > a language design issue?
> 
> The latter.  It is the most derived class that must construct all
> virtual base classes, even indirect ones.  If it doesn't, default
> constructors are used, even if other base classes specify constructor
> arguments for virtual bases.


If I understand what you are suggesting correctly then it is
_impossible_ to get the desired behavior with the given code.

Since  template <typename X> class HoldX  has no knowledge of what
base classes  X  might have to even provide constructors to specify
them in its "mem-initializer-list".

In other words all template classes with template parameter classes
containing virtual bases classes (direct or indirect) are restricted
to using/initializing only the default constructors of these virtual
base classes.  ???

I'm not sure about you but this strikes me as /quite odd/!

If this is true ... then there is a HUGE limitation in the C++
language!


I don't think this makes sense at all.  Can you please provide us with
a reference to this behavior/limitation in the standard?


The only section I find is the following:


[12.6.2  Initializing bases and members]
//
//   Initialization shall proceed in the following order:
//
//   - First, and only for the constructor of the most derived class
//     as described below, virtual base classes shall be initialized
//     in the order they appear on a depth-first left-to-right
//     traversal of the directed acyclic graph of base classes, where
//     "left-to-right" is the order of appearance of the base class
//     names in the derived class base-specifier-list.


Which seems to be supporting your point.  Which, originally I wasn't too
sure if it actually meant *all virtual base classes* (including direct
and indirect).

But the more i read the above it is more clear that that is in-fact what
it means.


The only possible way of getting the intended behavior from the given
code would be to write a "wrapper" class to be the most derived class
and specify the desired constructors of the virtual base classes in its
"mem-initializer-list".

Eeeek!  Talk about a hack, error prone, tedious and a maintenance
nightmare!


Please correct me if i'm wrong.  Originally I was going to discard this
thread but now it's become more interesting :>


patrick
--
when i grow up i want to be a famous rock'n roll guitar player
      -- steve vai