Re: [GD-General] Eiffel

[Jesse J. <jes...@mi...>]

At 4:56 PM -0500 12/27/01, Thatcher Ulrich wrote:
>On Dec 22, 2001 at 04:30 -0800, Jesse Jones wrote:
>>
>>  Unfortunately MI has a bad reputation in C++. Even people who should
>>  know better like Scot Myers rag on it. But essentially all of the
>>  problems with MI in C++ are because of virtual base classes. So how
>>  do you avoid virtual base classes? By avoiding diamond shaped
>>  inheritance hierarchies.
>>
>>  One way to do this is by using mixin classes. You have your main-line
>>  classes like Widget or Monster or whatever and then you have mixin
>>  classes like ReferenceCountedMixin or ObserverMixin or whatever.
>>  Mixins only descend from other mixins so you never wind up with the
>>  diamond of death.
>
>I've been pondering this.  I've always depended on virtual inheritance
>for effective mixins.  For example, how do you implement smart
>pointers to mixins of ref-counted classes, without virtual
>inheritance?

I assume you're talking about a smart pointer class that requires 
invasive changes to the object it's pointing at. For example, a smart 
pointer class that maintains ref counts and requires that the object 
have a common reference counted implementation. In general I think 
it's better to try to come up with a non-invasive design.

However I have dealt with problems like this. For example, I used to 
have an Invariant class that allowed for DbC-style invariant checks. 
The class had a virtual Invariant method and a counter that was 
incremented when entering a public method and decremented when 
exiting a public method. But mixins still wanted to call the 
invariant so what I did was dynamic_cast the mixin this pointer to an 
Invariant* and call the Invariant method if the result wasn't NULL. 
However I've fairly recently dropped the Invariant class.

   -- Jesse